So, in Guild Wars 2, they have a neat little system that allows you to collect miniature versions of characters and enemies. Considering they use the skins of characters and enemies in the game already, just miniaturized and set to just follow the player around, I imagine they would be less of a headache to implement than some other things, and make players excited for opening chests again once they collect the reds they want. Anyway, I wanted to suggest using the VT2 enemies and characters to create a series of collectible miniatures that can drop from the chests in legendary with similar frequency to red weapons. (maybe also include “shiny,” rare versions of some of the mini-pets like in pokemon)
Here’s a link to an article on Guild Wars 2’s implementation of mini-pets.