What if we could get action figures or Items, Armors or Mounts that allowed us to act like the bosses. For example, we could get a C.R.E.E.P Backpack. It could be very expensive, and the only way we could get it is to visit Playtopia and visit the Toy Maker. Let's say if you click M1 you spray green and black pixon like projectile and if you click M2 you create explosions below your enemies feet. If you press Ctrl the boss music will play. It would be very cool to upgrade CREEP so his projectile could summon creepers, charge nearby creepers and create explosions below your feet. Because it's a toy it has 0 defense. To use it , equip it in the chestplate slot and have open hands.
Interesting suggestion. It could be part of hardmode, bosses are harder with more attacks and health but have one special drop that is better than the normal items, assisting you in the hardmode progression (silverfoots mace, the eye of shadow lord, graw tablet (mini graw mount with push affect on mobs) and, of course, the C.R.E.E.P. backpack)
EDIT: Now that I think about it, isn't that a cannon creep has mounted on his back? The mini explosives shot up seem to come from that "cannon"