Seems odd, but yesterday I was thinking about something similar.
I thought it would be nice to have a general PvP - the one that matters for ranks - exactly as we have, and then Rare, Epic and Legendary category, where you can compete only if your items are at a lower or equal rank than the category's rank.
That way everyone will have at least a reason to play some underdog item, in particoular the one that stop their evolution at rare/epic/legendary stadium (e.g. Golem, Power Bottoms, etc).
Some of these items are nice but no one who wanna be competitive use 'em, since they lose vs mythical ones. Also, we can have a dedicated ranking system that resets itself every week, with some minor reward (e.g. grey boxes or something like that, just for funz).
It shouldn't be too much of a pain programming wise, since TS can take its PvP system and simply add a check on items stadium; here you have 3 more categories.
Not sure if a Common category is needed, almost every item can be upgraded to Rare with very few exceptions.
May post this on suggestion later if it seems appreciated.