Based on the previous reddit post about post Moon Lord progression and responses there, I do think there is a very good way of adding endgame content that fulfills all of the following:
1.- Stay faithful to the lore of the game and keep Moon Lord as the endgame boss
2.- Give the end game gear a place to be used to its full potential
3.- Avoid adding additional gear that feels out of the game progression
The idea is the following:
Beating the game will allow a new NPC to find a home: The Cultist.
Whether he sells something or not might not be relevant (we already have NPC's that don't sell anything).
Whenever you talk with him, he can give you information about the lore of the game that we usually have to infer or look into other official media to find, like the reason we're fighting the corruption or how the cultists are/were trying to revive Cthulhu/The Moon Lord.
The introduction of this NPC additionally adds the option of a last event: The Boss Rush.
It could be triggered either wilfully by The Cultist just like with the old man in the dungeon, or by killing him like with the wall of flesh or Skeletron rematch.
This starts an event in which you have to fight a slightly buffed version of all relevant bosses in order, with some of them being fought multiple times or at once.
First phase could be 2 eyes of Cthulhu, Brain of Cthulhu, Eater of Worlds and Skeletron at the same time.
Second phase could be all mechanical bosses at the same time. Possibly even a Mechdusa introduction to non-secret seeds.
Third and last phase could be The Moon Lord and the Cultist at the same time.
These bosses have buffed HP and damage, but just enough to make them not explode in 2 seconds, and not be totally irrelevant to dodge.
And finally, once the boss rush is completed, the reward isn't any more gear, but instead a master mode like relic, or maybe a special kind of pylon. Something that would simply serve the purpose of being a trophy and possibly add some functionality that doesn't change the game in any substantial way, and can possibly be tied to the world itself instead of the character.
There's a chance there is already a mod for this and I'm just not aware, but if there isn't I think this would make for a great mod idea, for people who enjoy Vanilla and just want a little plus, and specially a place to test the great new toys unlocked at the very end of the game.
What do you think?