I think youre right. People are always trying to imagine the next reddit and its not going to be some clone like voat or whatever. Hackernews is actually a lot like reddit used to be but its not going to scale to reddits size without subreddits, or the like.
I dont know if anything will overtake Reddit for a very long time because of network effects. But discord is probably the best guess. Although I actually think people do want centralization. They want 1 login to 1 website that has everything.