I'd like it if there was a way to trade in lower level rarity items for higher rarity items. I have way too many low level heat crystals that just go to waste, I can't sell them and there is nothing i have that can be leveled up by those things. As for alchemy orbs, well, most low level weapons don't have any higher level counterparts, and most of those that do are not worth the trouble... It'd be nice if people could trade these lower rarities in for higher rarities, or at the very least be able to sell them.
I think being able to sell should be the limit. Trading for a higher level of it would throw you to 5* almost immediately. Or they could make an option to drop fire crystals, alchemy orbs, etc or just plain good 'n' all crowns.