I'm like 95% sure how the season is going to end and if I'm right we may see a very different version of the show next season and here is a few ideas of how I think the season, well the first half at least, should go.
Near the end of the third season Savitar does actually kill Iris because this Savitar is the youngest version of himself who hasn't yet been imprisoned in the Speed Force but because he does kill Iris, Barry creates the prison in the Speed Force that holds Savitar for centuries allowing him to carry out his actions at the beginning of the season. Then Barry will have to defeat the actual Savitar probably with Wally, Jesse and Jay. However I think that Barry, against everyone's advise, will travel back and save Iris from Savitar and while he'll still imprison Savitar allowing his future actions to be carried out, Iris will be saved and her name will return on the newspaper.
However because Barry will have broken his promise yet again the Speed Force will finally deem him unworthy and send the Black Flash after him and when it threatens to kill everyone Barry loves, Barry will sacrifice himself to save them and Black Flash will kill him and he'll be transported to the Speed Force to be punished for his recklessness. In the aftermath Iris and Joe are left devastated and Caitlin fully becomes Killer Frost. Not sure what will happen to Julian but I think HR will probably stay around as we do need a Wells and he's now stuck on Earth-1 as he's not allowed back on Earth-19.
When Season 4 begins Wally West / Kid Flash is continuing on in Barry's memory but HR is his only ally as Iris and Joe are still in grief over Barry's loss and Cisco has inherited S.T.A.R. Labs as part of Barry's will and has re-branded it as "Ramon Industries", and is starting to sell his tech-apps for public use. However because he has become disillusioned Cisco no longer wants any part of Team Flash's heroics. Killer Frost continues to linger around Central City and Kid Flash has occasionally come into conflict with her. Meanwhile HR has honored Barry his own way by creating the Flash Museum which contains display artifacts of The Flash and his Rouges and all his achievements, though obviously doesn't reveal Barry's name for security reasons. While many people praise Kid Flash as a hero others see him as little more than a groupie trying to replace The Flash, which irritates Wally. However Wally is secretly questioning if he really can fill Barry's shoes as Central City's defender. This is only magnified when Iris and Joe constantly try to discourage him from his destiny, fearing that if even Barry can be killed then Wally will be because of his youthful inexperience and brashness.
Wally's biggest challenge comes when Captan Cold and Golden Glider return and reform the Rouges consisting of Mirror Master, Weather Wizard, Top and Dr. Light. Because seriously why have we not had at least a 5 person Rogues team up yet, and when the hell is Cold coming back! Against all 6 of them by himself Wally falters and he finally listens to his family and gives up the mantle. However when Weather Wizard disobeys Cold's orders and attacks C.C.P.D. and hurts Joe, Wally goes right back to it but knows he needs his team. Cisco is reluctant but agrees to let them use Ramon Industries as a base until the Rogues are dealt with, but Caitlin refuses. Kid Flash and Vibe then confront the Rouges and are joined by a reluctant Killer Frost, only helping so she doesn't have competition herself, and after Iris gives Wally a pep talk, the three work together to defeat the Rouges. While Caitlin flees yet again Team Flash is reunited and is now aiding Wally's heroics in memory of Barry's. This doesn't mean Wally is the new main character because it would revolve around all of Team Flash trying to move on, and would kinda adopt Legends of Tomorrow's tradition of not having a set main protagonist but equally revolve around all the main characters.
I think the primary antagonist of this season should be Abra Kadabra as I am quite frankly tired of speedster main villains when The Flash has a number of Rogues who have big bad potential which includes Abra himself and Cobalt Blue. Doctor Alchemy could have been a fearsome threat on his own but instead they neutered him in favor of him being the lackey of yet another speedster. Abra as a sorcerer, either from his various technologies or as an actual sorcerer as he later becomes would be a much better choice of villain. Obviously the other problem is that Abra is from the future and almost all of Flash's villains are from the future and his motives aren't very big bad material, but they could work around it and redesign his character to make him more big bad level. I don't really know but I'm just hoping the next main villain isn't a f***ing speedster.
In addition to creating the Flash Museum which is now booming with popularity HR has also begun writing several best sellers novels based on his experiences on Earth-1 and Earth-19, and has finally found his place on Earth-1. However he has also pitched an idea for another book which is basically an autobiography of The Flash and his achievements as Central City's superhero, but because Iris was closest with him he feels as though she should be the one to write it. He even has a name for it "The Life Story of The Flash". However Iris refuses once approached with the idea still not quite ready to move on much less write about her superhero boyfriend's career as The Flash. This is also because of public rumors that Iris was romantically involved with The Flash given her history with him and that her being friends with him is basically public knowledge, which only causes her further distress. It's also revealed through recurring nightmares that Iris blames herself for Barry's death as he sacrificed himself to save her, and wonders if he would still be alive if she hadn't been with him. While deep down she knows that Barry probably would have given his life for her regardless if they were together or not, she still finds it hard not to hold herself responsible. However after seeing what so much grief did to Caitlin she tries so hard to stay strong and not fall off the deep end like she did.
Killer Frost would continue to be a recurring foe to Team Flash but because of what she means to them they often hesitate and try to reason with her, to no avail. But Killer Frost would often find herself forced into reluctant team ups with Team Flash against a common adversary but would ignore their attempts to reason with her, and would flee in time to avoid capture. Despite some members of Team Flash losing faith and while it would be shown that while Killer Frost is dominant, deep inside Caitlin's true personality is still there, screaming at her villainous alter-ego to stop her killing sprees. However Killer Frost often ignores her true self, deeming her the "weaker half", spitefully keeping her buried and trying to forget she exists.
While I do ship Wally and Jesse Quick there is two aspects of Wally West's traditional character I think need to be adressed: His relationship with Linda Park and his tendecy to fall in love with supervillains (Magenta and Jinx), or superheroes related to the villains (Artemis). The perfect way to tribute Kid Flash in this way is to have a flirtatious relationship between Kid Flash and Dr. Light, as she is a supervillain and is a Linda Park so it's double win. What would be hallarious is if Wally doesn't actually take her to prison, but instead just keeps returning everything she steals back leaving her frustrated and confused. I think it would be funny story material for Wally and Iris as Earth-1 Linda is a friend of her's, and now Wally is crushing on her villainous counterpart. This could also give Wally and Cisco something to bond over, as Cisco has crushed on Golden Glider and now Gypsy. This one is a bit of a stretch I admit but I think it would be awesome.
I'm not sure about Julian Albert surviving this season but I think it would be awesome if he did and became the new Dr. Wells of the team. Obviously since Eobard is dead/erased/whatever and can't be part of the team now anyway, and since Harry can't keep travelling between Earth-1 and Earth-2 and HR isn't a scientist and if Caitlin is turning into a villain like I think, they need someone who knows science and meta-humans. Also if Caitlin is going to become a villain then this may be the reason Julian, if he survives, stays on to help find a cure for her condition, given their blooming romance at the moment.
The next part adapts material from The Return of Barry Allen storyline which if I'm right and Barry is gonna die at the end of Season 3, then Season 4 would be a prime chance to adapt that story and it would allow Grant Gustin to stay on the show without Barry actually being in it.
Throughout the first few episodes of the season while Wally would be continuing his heroics Barry is revealed to still be alive in post-credit scenes but appears to have amnesia until finally he returns home and everyone is overjoyed at his return. Barry claims that the Speed Force punished him by keeping him imprisoned for centuies to fully understand the depths of his crimes. After waiting for thousands of years it finally released him and perimited him one last time jump to the present but as a reminder of his constant recklessness it has now cursed him with red lightning, like it had with Hunter Zolomon. With Barry back Wally happily returns to being his student and they fight crime together once more. While Iris is overjoyed to have her boyfriend back she notices subtle changes in Barry's personality such as increased agression and is suffering recuring nightmares, and Iris worries that his centuries of imprisonment in the Speed Force has driven him insane. Joe also begins to share this view when Barry begins lashing out at his superiors at work. Furthermore Barry and Cisco end up in an arguement that comes to physical blows but when Cisco touches him, he gets a vibe of a blonde haired man who resembles Eddie Thawne but in a lab coat and in a more teched out enviroment and hears someone jokingly call this person "Professor Zoom".
When facing threats together with Wally, Barry is more ruthless and even kills a bank robber and Wally confronts him, only for Barry to claim that Wally is stealing his legacy and is unworthy of it and tries to kill him. While Wally is assisted by Jesse and Jay, Barry kidnaps Iris and drops off the grid. While in captivity Iris realizes that the man she's with isn't Barry and he finally reveals himself to be Eobard Thawne, but this version hasn't even encountered Barry for the first time because he hasn't met Barry yet and explains his story: In 2175 when he was 24 years old during the Thanagarian invasion of Earth, he saw streaks of red and yellow lightning whisk around him which saved him from harm and after discovering the steak belongs to the 21st century superhero, The Flash, Eobard grew obsessed with the hero and wanted to be just like him.
After replicating his powers and becoming The Flash of the 22nd century Eobard wanted to meet Barry but traveled back in time only to land in the wrong time period, this current present. While researching in the Flash Museum for the Flash's actual time of origin he discovered it was his destiny to become his greatest enemy the Reverse-Flash, after seeing a statue of him with his real name name branded on it. With that he also realized that the red streak that saved him as a young man was not The Flash, but really his own future Reverse-Flash self, saving him from The Flash for crimes he hadn't committed yet and was idolizing the very person who was trying to kill him.
With this Eobard grew resentful of his idol but also scared to embrace his fate so he found Barry's corpse and replicated his appearance, like he did with Wells, to blend in as him to hide from history and to discredit Barry's legacy with Eobard hoping it will change his future. However Iris is disgusted to learn the man she's been with these last few weeks was in fact her still-dead boyfriend's archenemy, though Eobard admits to having become genuinely fond of Iris, much to her disgust. Eobard also reveals how he knew so much about Barry despite the historical archives having virtually no information on The Flash in this time, and reveals the one possession he took with him: A novel called "The Life Story of The Flash" written by Iris West-Allen which is a centuries old version of the same novel HR pitched for Iris to write. However when Eobard reads a few pages of it to her it's revealed that the Crisis in 2024 still happens even in his time which indicates that Barry may not be dead after all, further indicated by the fact that it is written by "Iris West-Allen" not "Iris West".
Eventually Wally locates confronts Eobard with Jesse and Jay but the evil speedster proves much faster, but Wally also realizes his failure to beat him is because of his lack of self-confidence as he is now fighting the one enemy even Barry has never truly defeated. However Iris convinces Wally to stop letting his fear and doubts get hold him back nor what other people think of him and after finding his inner strength, proves fast enough to outrun Thawne and they have an epic street race. While that happens Jesse uses her speed to create a Speed Force portal. When the portal is readied Wally lands a super-sonic punch directly into Eobard's face causing his severe disorentation and finally Jay throws him through the portal, and he is forcibly returned to 2195, or whenever Eobard actually comes from. However Eobard is now suffering severe amnesia and barely remembers anything, except his trip to the muesum and his impending destiny. Though eventually Eobard will come into conflict with Barry for real and fight him for years and one of these encounters will jog his memory enough to remember everything he forgot, and thus Barry's name which will lead to him going back to kill Nora.
Afterward everyone resumes their duties with Team Flash while Iris tells them what Thawne said to her, that Barry is still alive by 2024 at least in his timeline. Furthermore they confirm the newspaper still reads Flash Missing Vanishes In Crisis in 2024 and is now written again by Iris West-Allen. With this everyone begins to have hope that Barry will return and everyone is able to move on, now determined to keep the city safe until his return. Iris also becomes more open about writing the novel but decides to wait until Barry has truly returned. However in a mid-season finale twist in a post-credit scene the actual present day Eobard is revealed to have the actual still-alive Barry captive in a cage, having somehow revived him, as retribution for imprisoning him in Flashpoint and indirectly turning him into a time remnant and being hunted by Hunter Zolomon.
I'm not gonna write a whole "how the season should be post" like I have before, I just wanted to show a few ideas and one major one.