Endshards is a mod for those players who are bored in the endgame. Sure, building has its appeal, but you want challenge! Adventure! Well, you need not look further. Endshards extends the late game by making it more difficult to acquire gear beyond diamond, each set requiring you to defeat a boss. This includes the existing Netherite, which will require defeating the Wither. If that seems a bit mean, don't worry, Netherite and the new gear added by Endshards all have special abilities to make your gameplay more fun!
Finally, Endshards adds a new dimension, the Nightmare Realm, somewhat inspired by the Spirit World from Witchery. This new dimension is still in the works, and even I don't know what will be in it yet, but I can promise a variety of biomes and an exciting new boss fight!
More info on the wiki.