Re: the hosting of topics/subreddits in the DHT, I've done quite a lot of research [1] into a very innovative yet not well known P2P publish-subscribe network design [2] from some Norweigan computer scientists that removes the role of hosting for nodes not interested in a topic, even designing a decentralised microblogging platform on top of it [3].

It's called PolderCast, and the way it works is that it models subscribers of a topic as a ring, and then uses 3 different overlay networking modules to efficiently constructing the network (a basic gossip one, one that finds nodes with intersecting interests, and then a final one which constructs the neighbours for a node in the topic ring). Thus, only subscribers of a topic are responsible for hosting, in contrast to a DHT where every node would be (even for questionable content that they may not agree with).

[1] http://liamz.co/wp-content/uploads/2015/03/Computer-Science-...

[2] http://acropolis.cs.vu.nl/~spyros/www/papers/PolderCast.pdf

[3] BitWeav http://liamz.co/wp-content/uploads/2015/03/whitepaper.pdf