That would be awfully fun. Mabinogi has something pretty similar and all.. actually we could stand to take a few leaves from their book in general. NPCs remembering you and behaving differently accordingly, clothing systems...
Hm, but, as for vaguely original ideas and the subject at hand? I don't know about a golden saucer. Even in FF7 it kinda clashed with the setting, here a theme park of any sort would be really uncomfortable...
But actual little minigames? Hm.. definitely I'd think, though I'd say the most important one would be chocobo rearing and racing. If we're going with other FF's ideas though.. what about something like defending the old MAKO reactor in FF7? XD I was always a bit sad the Golden Saucer didn't let me replay that one.