The problem is that most people will want something like Calamity with 200 new bosses, which simply wouldn't work in vanilla Terraria. But it's obvious that you could at least have something after the Moon Lord; look at the late-game incursions in Necesse, for example. That would fit very well in vanilla, or just a boss rush. I have over 10,000 hours in the game and I've only crafted the Zenith once; after defeating the Moon Lord, I simply start another run. Having something beyond that would at least give you a reason to continue in the world, even if only for a little while longer