CW justice league With Arrow and The Flash both introducing many heroes, I feel that it's all leading to a large superhero team up. With the movie and the Tv universes 100% separate and the real justice leaguers (Batman, Superman, Wonder Woman etc) obviously never appearing, the CW will be forming their own.
The name: Obviously "Justice League" won't and can't be used, so I think the group will only mention something boring and neutral like "the team" or "the group". Similar to Smallville never officially naming their team of heroes.
1.Green Arrow Oliver will play the "Batman role" as the teams leader and tactician. He most likely brings the team together in the first place, and the "Arrow Cave" most likely is used as their base.
2.The Flash Barry will play a "Superman role" as the team's heart and as the "powerhouse". He will also provide more of a light comic relief to the team.
3.Black Canary Hopefully Laurel will be better trained and have some variation of her canary cry, by the time of the team up. She takes Wonder Woman's role as the badass female member.
4.Firestorm This character will be similar to The Hulk in the Avengers, providing both muscle(Ronnie) and intelligence(Stein).
5.The Atom I highly doubt shrinking will be used as the budget won't allow it(despite a surprisingly great looking CGI Grodd) so I think his armour will be more like "Iron Man": enhancing his strength, allowing him to fire electricity, maybe a force field etc
The next members (if any) I'm not sure about, it could be any of the following : Felicity and Caitlyn: Providing technical support similar to Oracle like they normally do.
Diggle: Maybe he finally gets his own costume however this is the unlikely.
Arsenal: More of a sidekick (Robin isn't in the Justice League) still likely to help out.
Katana: Maybe Oliver convinces her to "suit up" and help protect Starling/Central city.
Maseo: Due to his relationship with the league of assassins this is also unlikely.
Vibe: Cisco could either give himself a tech suit or become a metahuman.
Speedy/Cheshire/Artemis: Unless Thea is killed or leaves the show it's pretty obvious she's going to join Team Arrow eventually.
Wildcat: Ted could make a surprise reappearance, however his character is a little lacklustre so far.
Deadshot: If Waller puts the team together then she could place one of the Suicide Squad members on the team for insurance.
Vixen: It's possible she could get a live action appearance after her animated series, however this wouldn't be for a long time.
The Villains From least likely to most likely
1.The League of Assassins: Oliver may need the teams help to fight Ra's and his army, however with the season already filmed and with Oliver already having Diggle, Roy, Felicity, Thea, Laurel, and possibly Tatsu, Maseo and Malcolm, he has more then enough back up without them. Plus the "Justice League" may be too powerful just for the League of Assassins.
2.The Rogues: Flash's villains will obviously be teaming up either at the end of this season or at the beginning of the next. Depending on who is threatening Barry, he may need backup, however The Flash needing help of "The Team" downplays his own heroics. While it would be cool seeing a fight that includes powers vs powers, The Flash can fight his own battles, however The Rogues may cause too much destruction for just Flash alone.
3. Some ARGUS Foe: The most likely option is Amanda Waller forming this team of heroes, to fight an enemy that is too powerful for ARGUS or the Suicide Squad to stop on their own. Amanda would probably confront Oliver with the problem and he will form the superhero team to counter it.