I haven't gotten much into the NES/SNES FF's but, Final Fantasy Legend 3 (Game boy), in fact all the Game boy psudo FF's, had tons of extra slots for armor. FFL3 had one for weapon, shield, helmet, body, gloves, and shoes as well as two extra slots for bonus accessories.

I think that including a shield might be useless though, unless you had one and two handed weapons for each character so they could use a shield, but that's getting a bit too much like Diablo.

I sure liked collecting full sets even though there was no extra effect if you did so. I think you could almost make collecting special sets or parts of sets optional side quests. With all the work you put into sets though, you should gain some special ability with each set. Like if you had one character with all Holy armor they would be immune from instant death attacks, or someone with Dark armor could absorb dark spells (duh). Even ultimate magics like Ultima or Flare Star or Doomsday could be taught to magic users who have the appropriate complete sets.

You could also do weapon sets. Like a long sword and a matching short sword or dagger. Maybe.

I think changing the in game look of the characters would be cool but too complicated and a bitch to program. All the extra surfaces would have to change every time, ugh. Of course you would not want change the appearance of the characters head because that's how you recognize people. Even if the bodies are different and you had the ability to see the armor change so everyone had, say, silver/gray armor on, then all you have is a bunch of silver/gray blobs.

To summarize that last paragraph: Imagine all your characters looking like the Knights of Pluto (FF9), how are you going to tell the difference?