I expect for Helen Slater to play her version of Supergirl. She's already in the show as Kara's Earth mom
I think I'd rather that some of these appearances were seeded into the episodes leading up to Crisis. Because if they try to squeeze them into the five episodes (or worse yet all in one episode), they aren't going to get enough screen time.
Marv Wolfman hired as a consultant on CRISIS.
https://comicbook.com/dc/2019/08/04/...-marv-wolfman/
What I would like to see, but probably will not happen, is to have an epic opening sequence, like you see with Japanese animation, to celebrate bringing one of the first ambitious comic book events to screen.
Example:
Thank you for your time and patience.
The first time Barry went from his Earth-1 to Supergirl's Earth-38, they used old footage from some of the old TV shows. I think that's what should happen--a montage of film as the Monitor moves through the multiverse. No need to create CGI images of Christopher Reeve or Adam West. They just need to get permission to use a second of two of film for each of them. However, that might be impossible if there are certain legal agreements that don't allow it or if the money involved is astronomical.