I assume there will be some kind of buildup to Crisis, maybe from the beginning of the season or a few episodes before.
As far as what will happen. They might throw us a curve but all signs point to Arrow being killed off. No way they will kill of Supergirl and Flash like in the comics for obvious reasons. I mean I guess they could have Wally West take over for Flash, which on paper could be interesting. But I doubt the network would love the idea of taking the star out of their biggest of the franchise without him wanting to leave especially, and Supergil obviously couldn’t continue so she can’t be being killed off. So my guess is Arrow will bite it.
As for who if anyone will be killed off (imagine some won’t make it if they want to live up to the REAL Crisis.). I would guess that one or two of the Legends might be killed off. The cast has gotten huge and they like to rotate characters out. So would seem an obvious way to do it. Although some might be written off dead or otherwise in the finale, could see the series is setting up to write off Ray and possibly Nora with him.
I also could see them killing off some of the Flash cast seeing as how rumors of most of the cast leaving have been circulating. It’s possible they are just sticking around next season until the crossover. Wells would be a likely one to write off with Crisis. If like the comics they combine the multiverse (seems a likely outcome) then the whole Wells is a different earth’s version each season couldn’t really keep going. So if they do that either one lone surviving Wells will be a main character going forward or they will write him off and end his storyline. Sisko I could also see being killed off, as rumors of him leaving were rampant and the signs were there. They seemed to be setting that up with the final episode, but never actually said he left the team. He might appear scarce until the crossover.
I also assume they will combine the multiverse. Naratively I don’t like the idea, but I am sure that it would fix the Supergirl is on another earth problem the show has had since it came to CW and they got stuck with that decision they made when it was on another network.
I also assume some characters from past DC shows will appear. I think a lot of fans want to see every past DC show and the DCU shows all represented, and both financially, rights wise and logistically I don’t see how they could ever begin to pull that off. But I think one or two characters from past DC shows will be making an appearance. Although would be fun if they did some bit like the various earths looking at the skies turning red and CGIed in shots from past DC shows into it somehow in a quick montage. But that seems like it would be costly and have logistic problems too.
I do hope it doesn’t dissapoint too many people. The one problem I think CW might have from announcing something that big that much in advance is fans have had time to theorize and in a lot of cases are building up ideas that the show can’t possibly pull off and will end up going in with unrealistic expectations. But I can’t say I am not looking forward to it.