The only way I'd be for 'boss upgrades', or a boss 'shop', is if it was handled carefully and cost a decent amount of boss coins rather than points because we already know what happens with the normals, minibosses get spammed every round if someone's bored. They'd have to end up being minor buffs or tweaks like Ephi taking less knockback, although I find that shouldn't be something people buy rather than actually put into the game since every game turns into "let's see how long the rocketmen can stall". My ending note is that these rounds don't even take that long, so unless these upgrades actually do something, it's just a point sink. It would be most unfortunate if something like this were to not have a lot of thought put into it.