A New Nether Boss suggestion : the Bedrock Golem

But a secret one. No communication on it ; leave the players spread the rumor.

We know players always make up their way to the Nether Roof right ? How about we toy a little with the community for breaking the laws of Minecraft's Physics ?

Here's the idea : A Boss that triggers its spawn when the player gets above a certain Y value and will start attacking you after a certain time. It could physically spawn from the bedrock, using the Warden's animation (emerging from the matter), within a radius from the player, once the timer runs out. Truly terrifying. Or, instead, appear as a "pile" of bedrock, a structure, within a radius from the player, then rise up once the timer runs out. That way, you're preparing the player for it, because it would build up some fear instead of jumpscaring the player.

The Bedrock Golem would be a unique world boss. There's only one in each world because he's the only "living" creature roaming the Nether Roof. Once you've killed him, the game will register your success, and forbid any other one to spawn. As for his drop ; it would only be a Collectioner's item, a decorative item ; his beating, bedrocky, heart.

The reason behind its behavior would be that it cannot stand anything modifying its bedrock realm. Any block, or creature, he spots, he'll destroy it. That way, you also, as the player, can buy you time by placing blocks between you and the Boss, in order to flee, build your nether portal, wait for help or brace yourself for the fight.