It seems kind of a waste that once you gain access to the better armor and weapons the old ones just kind of sit there, unused. I think there should be an incentive to use them after they've leveled up fully- perhaps extra XP after each match that you use them- a trade off for playing with weaker gear. This could help slighlty with balancing issues as well.
Another idea is to fully level up the weaker gear for a token reward.
I'd make it so that any armor piece in general can have let's say a gem that takes the stats of another different piece and applies it to the armor. That way anyone can mix and match armor sets if they like the look but don't like the current stats.
This of course would take place after you've unlocked all armor pieces in the game and have them all at level 3.