Something I STRONGLY hope WON'T happen. I like the multiverse the way it is...you know, multiverse-like.
I honestly don't know. I'm only speculating. But this is a chance to start to change things around. Do things that they didn't think of when Arrow started because they had no clue there would be a shared universe. If they don't bring Kara to Earth 1, I wouldn't mind Earth 1 getting its own Superman, with Kara still in space.
I don't expect, nor do I want, a faithful adaptation. I just want something awesome. You can't even do a faithful adaptation. Too many characters missing.
Things I want to see--
I want to see John Wesley Shipp return as Flash90. I want to do a REAL update, not a two second cameo. Make some actual references to that series. Have him play a role in a story arc that takes place on The Flash before the Crisis. Don't make him expendable or kill him. They disrespected the character last time. Change that.
Leave Superman out of it unless they actually use Superman, and not write him like they have been. I'd rather not see Superman at all than to see the character continue as is.
So beef up Supergirl's role. We aren't going to see her die in this Crisis and we aren't going to see Barry die either. In fact, don't kill anyone too big. Maybe a Legend other than Ray or Mick or Sarah.
On Arrow, the whole season should be Oliver's journey with Monitor.
The best thing about Supergirl's finale was that the Monitor was clearly going to revive Lex. Lex Luthor was the best thing about Supergirl since it moved to CW. It would have been very foolish to kill him, so having this out is great. Obviously, he has a role in COIE.
I'm fine with this being general concepts and not a faithful adaptation.