I liked how the mask vanity slot was implemented in DL, and I think it would work well in PL also. Mostly I want to be able to wear my valentine eye patch along with the aura without obscuring the aura, or wear it with another vanity helm with both showing (like dragon mystery helm vanity and eyepatch vanity). A floating vanity slot would be great too, so players with the aura vanity or xperia play floating vanities could have them float above their equipped gear or helmet vanities.
A similar system could be used for the torso, so wing/backpack slot would be separate from the vanity armor slot. This would allow wearing MFA with plat-purchased wings, for example. It would also make room for many kinds of back vanity: backpacks, quivers, monkeys, stegasaurus plates, etc., without obscuring or replacing equipped torso gear and vanities.
The main reason I began thinking about this is because I think players don't customize their characters as much as they would otherwise, because it is too much of a loss to break their elite vanity set bonus. I think that the players who have put in the effort to cap for all the elite vanities should still have options to individualize their character while the set is equipped.
One last thing, while i'm at it...
wouldn't it be cool if big luck could sprout through equipped helms? This might also be possible with it sprouting through an equipped vanity helm if the big luck were changed to the floating vanity category instead of the helm vanity. Then players wouldn't have to choose between big luck (which they paid for) and the vanity set bonus (which they paid and/or worked for).
I don't know if these ideas are feasible; not being a programmer, I just implicitly trust that STS can make anything!