If Supergirl is the Superman in the Arrowverse could they not use a old Supergirl in this storyline. I don't know the comic so I don't know how important it is for one of these characters to be old.
I don't know if I would go there.
Someone else had a very good idea to give this the justice it deserves, and that would be to cut short all four shows next year and make this an epic miniseries rather than a 3 night thing.
By cutting the seasons, the actors are not increasing their workloads.
This way, the story is done right.
I would not use an older Supergirl in the role of Kal-L. For one, this older version wouldn't have any reverence. Kal-L is the Action #1 Superman--the first.
There's nothing like that. About the closest you could get to what Kal-L meant would be if we lived in a world where Christopher Reeve never got hurt, and was in shape enough to put on the cape, AND would be willing to do this, AND would have already made a crossover appearance.
If they are truly going to do this right, they should set it up right. More multiverse stories--at least on Flash and Supergirl.
Let's get to know other versions of our characters, including a REAL crossover with Flash90.
The biggest reason to do COIE is to reset things and move in a new direction, so before Crisis begins, all storylines must be resolved as if the shows were having series finales.
What makes Crisis important to me is what they are going to do when it's over.
Freshen up the shows. Maybe reboot some things. For me, and I know it will never happen, but the best thing they can do is follow up on that John Stewart reference and have Diggle emerge from this story as a fully trained Green Lantern.
Maybe shuffle some casts around to keep it fresh--especially if they end up on one Earth.
It could allow for recasting as well if needed. COIE is big--let the end results be big.