Boss rewards

It would be cool if you would get sort of a power up from the already existing bosses in the game (Elder Guardian, Ender Dragon, Wither, maybe Warden in the future). After you had killed them you could go to a block and "sacrifice" a special item you got from them. After you had done this you would get a permanent buff, like after the sacrification of the elder Guardian item you could swim a little faster in the water forever. This would reward the boss fights more, because now I don't have any reason to fight the Wither or go into the underwater temple.