So I'm not sure what the dev/community stance is on "armor" or similar wearable item systems, but these are my personal thoughts on what could keep me personally(emphasized for good reason) around for-pretty-much-ever.
Aesthetics for characters. Could be armor, capes, maybe even just hats. But wearing something to distinguish end-game players from others has always been one of my favorite MMO systems.
Titans, if they are still planned/in the game sound like an awesome end-game goal. I would love to be able to go with a guild to hunt these monstrosities and reap their rewards. Perhaps some worldbosses could be stronger than others, requiring materials to be acquired from their lesser counterparts to vanquish. Idk I just love monster hunter
Maybe planets that are incredibly difficult to survive on without a group of adventurers loaded with diamond slingbows and grapples, but have new block types and materials not found anywhere else. Think about being able to show your prestige by building with block types/textures/tints that very few others have. Would stimulate the hell out of the economy as well, and provide a reason for end-game players to interact with players still on the starter worlds that visit towns!
I think there is a lot of potential with the complexity of the world generation in Boundless to have designed/natural worlds slowly come into existance as the game grows that provide players that have conquered all else to find new and exciting adventures. I don't know how well the idea of "raids" or anythign like that would fit in, but certainly new planets with new worldbosses roaming about could provide for endless fun.
For me personally endgame for all MMOs is usually about the aesthetics, whether it's the buildings or the wearables. I farmed the hell out of transmogs while being the best healer in my guild. Guess which one i cared about more? I think skill trees are the dankest thing humans have ever created, but I like to know that others can see my accomplishments and how little life I have outside this game
EDIT: There is a 'Titles' tab in the test version that i just noticed. So there's definitely that to look forward to