Took me a while but I think I’ve figured out the mystery of Savitar, assuming he we ever actually find out his identity this season, and I think he’s a time remnant of Barry from a now deleted timeline. I think there was a timeline where Barry never created Flashpoint, yet the events of Season 1 and Season 2 remained intact, yet he lived a much more embittered life where he never recovered from Henry’s death and became a much darker hero. In this timeline he still knew Jay Garrick and Earth-19 Harrison Wells but Wally never became Kid Flash while Caitlin and Cisco still became Vibe and Killer Frost.
I think there is a timeline where Barry never made Flashpoint, but became more cynical as a result.
I think in the 2024 of that timeline something endangered Central City, possibly because of Eobard Thawne as the future newspaper reads, so Barry created a time remnant of himself to be used as a sacrifice to save everyone else as he considered time remnants mere copies of the actual speedster. However something went wrong and while the crisis was averted the time remnant itself lived and was now marooned in the present, a timeline where everyone he cared for already had their own Barry and he was cut off from his own. As a result time remnant Barry, being as cynical as his other self, came to resent himself for being plucked from his own timeline and condemned to live a life where he was merely an echo of his other self and was unable to return to his own time.
If Savitar is a time remnant of Barry, it makes sense if Black Flash tried to kill him too just like he did Eobard.
Furthermore since he is a time remnant he may have been hunted by Hunter Zolomon / Black Flash as his existence is a paradox, being a copy of the real Barry of that time. Now hunted by Black Flash in an existence where there is another Barry living the life he wants for himself yet knows he cannot have, he may pushed over the edge of sanity as the real Barry of that time was already unstable. In his quest for vengeance on his other self I think he found the Philosopher’s stone and became Savitar, becoming fast enough to beat his other self, as their speeds would have been equal. However Savitar, already driven insane and boiled up with resentment wanted to destroy his other self entirely, destroyed Central City to prove his point. Knowing he couldn’t defeat Savitar and wanting revenge for destroying everything he cared for 2024-Barry used the stone to create a prison in the Speed Force itself designed to make Savitar live out his worst nightmares repeatedly in a never ending time loop.
I thik this Savitar hasn't yet lived through the events of Season 3 yet, which is why they can't change Iris' fate.
During 2024-Barry’s attempt to imprison Savitar I think they both ended up running into the past by accident to 2017, causing several timeline changes along the way, where 2017-Barry who already knows about Savitar from his own past interactions with Savitar’s future self was waiting for him. Savitar then killed the 2024-Barry and saw opportunity to torment Barry all over again by ruining the life of 2017-Barry of the new timeline, and killed Iris right in front of him. Then 2017-Barry finished 2024-Barry’s work and used the stone of his time to imprison Savitar in 2024-Barry’s prison in the Speed Force, which exists in his time as the Speed Force reaches beyond space and time itself.
I think Savitar used the Alchemy identity on other Earths in his attempts to free himself.
Then Savitar is then trapped for centuries and loses his mind several times over in his attempts to escape. This includes trying to breach himself to other Earths by manipulating people into finding the Philosopher’s stones of those worlds, empowered by his connection to the Earth One version of it to transcend into it's Multiverse counterparts. He then possessed those that found the stones under the guise of "Alchemy" and used them to try and free himself. During his quests he was always opposed by the speedsters of those worlds and while always succeeding in killing them, was never successful in breaking himself out and was always pulled back in. After accepting that nothing could free him from his prison or destroy it he continued to suffer in his hell, with only the desire for vengeance against 2017-Barry keeping him going. His previous exploits also caused him to become a feared legend throughout the Multiverse.
I think Savitar is now simply waiting for his own younger self's actions to be carried out, and will then destroy Central City as he promised.
Eventually Savitar sensed the creation of Flashpoint and Wally becoming Kid Flash and instead plotted to have Wally take his place in the Speed Force by forcing him to throw the stone into the Speed Force, giving him the means to escape and the means to trap Wally in his prison in his place. To do this he manipulated Julian Albert in 2012 in anticipation of Flashpoint’s creation in 2016 to become Doctor Alchemy to set up his plan, and when Barry returned he decided to torment him by recreating some of the Flashpoint meta-humans but his ultimate goal was to give Wally back his speed. Once that was achieved and he’d successfully manipulated him into falling for his trap Savitar broke free and is now simply waiting for his younger self to appear from his timeline and kill Iris so his own past isn’t jeopardized, and once Iris is dead there will be nothing stopping him from killing Barry and destroying Central City all over again.
So yea this has been stratching my head over but I think this is how Savitar could be a future version of Barry as I'm convinced he is but how Barry is going to defeat Savitar I don't know, unless he causes more timeline changes and risks getting the Black Flash sent after him?