Restricted Cast, who would you pick?

From a story experience standpoint, I'd caution heavily against removing Terra as a playable character. There is a connection you get to characters from having them in your team that doesn't happen by just having them in events, and that connection is crucial for making the characters care about the Espers, particularly Maduin.

I mean, as I've mentioned before, Sabin has pretty much zero impact on the overarching story. Once Vargas is defeated, his arc is over (and that's before he's even on your team). For every other scene where Sabin shows up, aside from the Figaro flashback sequence, his role could be filled by any other character. Even his scenario is more about Cyan than it is about him. However, Sabin SEEMS important to us, and that's for 2 reasons:
1. Blitz was a great ability that kept him in everyone's party.
2. If you're playing without a guide, he's likely the first character you recruit in the World of Ruin.
Because of that, players "wrote" a story in their heads in which Sabin was essential. That's the kind of connection people need to feel towards Terra.