I hate having to pick just one, but considering that it's unlikely they'd ever consider upping the available 'proportions' of your character I'll say an addon slot where you could put some extra items that don't conflict with others.

Essentially being able to do things like Lapis mask + Ultima horns

With that said a lot of the items I'd want are too combined. Like wolfliege chest would be great to just be able to take that damn tail off it and use it in conjunction with any chest outfit.