Since Midna is a decendant of the sorcers who were banished to the Twilight Realm, I don't think she has eternal life so she won't be around in another game. Unless there's another mirror of twilight or a portal of some kind, I don't think we'll be seeing the Twili again and I think it's for the best. Nothing good comes out of reusing a character when they're not set up to be brought back. Remember what happened to Michael Myers and Freddy Cruger?