I do like the idea. To some extent. I'd like to see Blitzball and other sportlike mini games from the series, like Snow Boarding and Chocobo Racing, just beefed up a little more. I think adding in regular sports like Football or Baseball wouldn't be that bad either. Just throw in some interesting fantasy elements to make it fun.
As for the characters... I'd love to see characters from the series. I mean, that, to me, would be the ultimate charm. Being able to make a sports team using a bunch of series characters. They'd all have their own individual special abilities, stat specialties, and so on. I think adding in new characters wouldn't be bad, just making some for this game especially. Also, just thought of this, but what about a character creation system as well? I think that'd be cool
To end... I think it'd be a pretty cool idea. If it was done right.