So a young Kara has been cast. And we know Laura Benanti has been cast as Alura. My guess is that the Alura parts of the show will be done as flashbacks to Krypton. We'll see young Kara learning lessons about life, about the House of El, etc. These will impact the present day Earth-bound stories. Since we will be heading back to Krypton, I have to think that Helen Slater and Dean Cain will be Lara and Jor-El.
Weissman has some geek cred having played a young April O'Neil in the live action TMNT.
I don't know of any other major characters that are left to cast!