Right now they are kinda useless besides the collection part. They are also a pain to use because you can’t “equip” them.
So, more than suggesting an item slot for them, why not do something cool?
Pokemon style battles with them would be awesome for example, and you already have battle animations for most of them.
Maybe give every mini a special attack with different effects and fight 3 on 3 or something so some strategy is needed and doesn’t become repetitive.
I really think making them more interesting would be a huge incentive to buy them.