I like the idea and I can +1 this. But why not take this even further since you calling this the 'Master' Bossing cape.
My idea on this is to have a Master Bossing Cape to have a extra droprate to all bosses. Not just the ones that we have the Boss capes for, but all bosses.
But to create this Master Cape, you need to get a certain drop from each and every boss to fuse it into a Master Bossing Cape
This way people will have to fight all bosses and not only the populair ones like Hydra and Vorkath these days.