Being born in 1987, the Justice League I knew and loved was a little different than what it is now. For better or for worse new characters came and left, old characters came back, but was that for the best?
With a JL movie right around the corner, the roster has to be carefully chosen, You have to examine the market, you can't just put the original roster in the movie DC has already shown us that by choosing Shazam and Cyborg as original members in every “confirmed” roster. These teased rosters leave out two characters that fans know and love, so far there has been no official mention (unless you count green socks) of The Flash or Green Lantern.
The former has his own new show set to debut soon, and the latter has a critical failure under his belt, which is one reason they may be excluded. These reasons aside, I want to share my opinion on how you can include those characters successfully without suffering from the shame of the GL movie or confusing the general audience with multiple incarnations of a character.
For the flash, we go Wally West instead of Barry. Barry is not half as interesting as Wally imo due to their personalities. Barry Allen is a man with un-shakeable morals, a forensic analyst who is as wooden as he is one dimensional. If we went Barry in the movie, I'm sure his personality would be changed to be more like Wally's.
We have already seen super speed in DOFP and it lends itself incredibly well to humor. Wally is very humorous, Barry not so much (except his running joke that despite being the worlds fastest man he is always late), the Flash could also serve as an intro for us as a viewer. Wally being introduced to the league and his reactions would mirror our own. Wally is scared of Batman and awed by Superman, just as the general audience would be. Wally is relatable in a way that Barry isn't due to his immaturity. I know Shazam is already on the team and is immature and therefore could be used the same way, but the coolest part of Shazam is nobody knows he is a kid, so if we tried to put him in this same characterization it would take away from some of what makes him such a great character.
So now we have Wally joining a newly formed League, Barry already came and went. Maybe he is being the Flash wherever he is but he is only mentioned briefly to cement his roll as The Flash before Wally. Wally is our comic relief, our quicksilver with a bit more humility. I would prefer his original red headed incarnation, although with the right casting I think that can be overlooked (although I would prefer it wasn't.)
For GL we can't go Hal Jordan. The character is the epitome of lazy writing, his whole story and personality are cliché. The man who overcame the fear of flying planes so that he was able to... well fly planes, is about as inspiring as a toddler getting over his fear of sitting on the big boy toilet and getting a Green Lantern ring for it. He is repeatedly touted as the greatest GL ever, despite the fact that it is explained that the most powerful thing a Lantern can do is create a construct, and Kyle Rayner is far superior at doing that. Not only would it be more interesting on screen (would you rather see a giant fist or a perfect image of a horned demon or monster throwing a punch in emerald green?) but it makes more sense that the ring would choose someone for their creativity.
The ring can create whatever the wielder can imagine, that is the power of the ring. So would it really choose a man who makes the bare minimum in terms of constructs? Hal Jordan more often than not prefers just a beam of energy, but the ring is not a gun, it is so much more than just weapon. Kyle overcame the weakness of yellow, and was the first Lantern to do so. He never even knew there was a yellow weakness, but was still able to effect it, how is that not better than a man who had the same ring for years and could never effect yellow, resulting in at least one instance in Hal getting his butt handed to him by the boy wonder painted the color of pee.
Yeah Hal gets his butt kicked by a boy painted yellow, whereas Kyle would have destroyed Dick Grayson due to the fact he overcame the weakness without even realizing he had done it.
Besides his obvious superiority in terms of wielding the ring, Kyle's story as the last Lantern is far more interesting and makes Kyle the only person in the universe with those powers, as opposed to Hal Jordan, who is just one of many, and despite being touted as the greatest GL there is no evidence except his willingness to die for the corps to support that. If we go the Hal Jordan route you will have to explain why the rest of the GL's would not be there to help the Earth in it's time of peril. Even if it was Hal's sector, he should still be able to call on his fellow GL's for help if he becomes overwhelmed. So unless you want to go into a long winded explanation as to why the other GL's are busy etc. it's easier to go with Kyle and have just one quick throwaway line where they ask Kyle to call more but he says he is the last one left.
In closing even Batman thinks Kyle is a better GL than Hal, and who are we to argue with Batman?