I believe decentralized Reddit can work quite well in this setup:

* Reddit-like engine that supports only one subreddit

* data importing tool that can fetch all the previous posts from Reddit sub and feed them to the engine

* mods of subreddits setup the engine on their own servers and migrate their subs' data there

* another useful addition - profile importing tool that can verify user, so you can preserve your username (via PM from bot on Reddit)

This way we're getting hundreds of small Reddit-like sites. If one fails for some reason - you can always fork it and try to attract audience to your version.