I like the OP's idea of character switching in battle, and also the suggestion that there needed to be more character diversity. Maybe instead of having the characters switch roles, I'd have given each of the characters one role, and paradigms were which set of 3 characters you had switched in at the moment. Sounds kind of like 10, now that I write that down, but I liked that aspect of 10.

Something a little more original I would have liked is no separate battle screen. Remember the first couple fights of FF10, how perfectly smooth the transition was between cutscene/world mode and battle mode?

Optional multiplayer wouldn't have hurt the game. Since I could only control one of my characters, why not let a friend who might be over while I'm playing have a chance to take control of one of the other characters in battle? I think some of the older FF remakes on Playstation allowed for a second player to control certain party members if you wanted.

The biggest aid to FF13, IMO, would be New Game Plus. I have all achievements, the game totally completed, and actually liked the story. There's NO way I'm playing through the game again, because it's painfully grindy, and sometimes just the gameplay is painful. I'd love to play through again when I feel like just seeing the story, but really, other than the story there's just grinding and tedious battles, and I don't feel like it when I could just YouTube cutscenes instead.