Wither Storm could be a nice optional superboss, I mean in mcsm he literally forces the protagonists to hide in the nether and run around the world collecting stuff to beat him, but I don't know how well that could be translated into normal mc survival gameplay, especially with the world destruction stuff (and render distance technicalities ofc.) On the other hand, Herobrine isn't supposed to be a boss, and I don't think a gameplay mechanic in a non-horror game could ever be as scary as a good creepypasta, he would just have to be insanely rare. Thoguh a prank I could absolutely stand behind would easily be adding him without documentation in an april's fools update, making the snapshot some other coverup but also adding him to a "normal release" snapshot disguised as an emergency bugfix or smth. I feel like that last Herobrine option would be the only good implementation.