Why do the instances need to be subs? It seems to me you just need NNTP extended to support voting and a few other features (most—probably all—of which could just be messages with new defined headers and, in some cases, no actual message body.)
I was thinking the other day. What if there was a browser extension that pulled in reddit/subreddit comment threads a la Alientube for the site or article you were viewing/searching. Literally anywhere you went on the internet for any hyperlink, you could check and see if there is a comment thread for that url. Decentralize the comment threads and related metadata onto something like IPFS and you have reddit but internet wide and you aren't limited to reddit's terrible search bar.
See this is why I don't do stuff like decentralized Reddit. What I proposed was my first idea but this is much better.
I wonder if decentralized voting for NNTP could actually be a good use case for some kind of block chain.