This seems rather counter intuitive as enjoyable is subjective, and everyone has their limit to how long they wish to play replayable content. My definition of endgame does not involve enjoyment. Now that’s not to say it should or should not be enjoyable, but again that’s subjective.
Personally speaking I like limits put into video games, I enjoy stopping points, whether that’s a soft or a hard stopping point. D4 and other games like it technically have both, but I’d argue everyone reaches the soft stopping point, as I’ve never seen anyone get BiS gear for every single slot, even on Eternal Realms for these types of games. PvP as you mentioned has a soft stopping point as well, seeing as there is no official competition for it with a leaderboard of sorts.
I don’t know of any content that will not become mind-numbingly boring after you’ve done it so many times, in any game mind you, not just Diablo-like games. Now if you have an idea of what your ideal end-game content would be that would keep you interested for 3 months on end, by all means share it with us.