Had a long post talking about how endgame boredom is caused more about eternal char youth than lack of content, but realized that's not going to change.
Instead, I'd like to suggest extending the infinite progression we have for stats also to skills. Basically, once you reach a certain level of development, the game becomes a slow number grind. That's bad game psych because the rewards are too imperceptible, to abstract or too distant, and motivation to play drops. That's also bad because when we get to that point we know there's nothing else to achieve. Even if we die, because the industries are still there to stuff newspawns with hq food and curios again.
The suggestion is that extra milestones be added focusing on endgame players. Skills, like stats, should be infinite. For example: a 1,200,000 LP skill that gives you unique crafting recipes, unique hearth magics, faster building speed, instant tree growth, efficient healing methods, and so on. If you're feeling hardcore, you could also add endgame skills that simulate old age, like one that brings massive crafting bonuses, but reduces 80 of your perception and constitution.
The point is that, if old age isn't a thing, we should not have the feeling that our characters have reached the end and done all they could. Endgame skills would still give people something to look forward to. Characters with such skills would be useful for all playstyles, with their unique items for trading, unique combat skills, or special homesteading buffs for the hermit. And since they are so expensive, they would retain a bit of new world mystery, as we wouldn't know how many of those skills there are, what they do, and what they interconnections are.