I'll quote this post here:
I wish that more work was being done on creating decentralized, uncensorable, but also usable forums. I would love nothing more than to be able to shut down bitcointalk.org due to some decentralized solution making it obsolete. Find me a 501(c)(3) nonprofit working on this, and I'll donate to it.
as far as i can tell, you're not going to get something closer to that wishlist than something like steemit. so it's already a solved problem. steemit is a huge forum. it's only weakness is the main interface is a website but that doesn't mean someone couldn't code a software application that could run locally off of the steem blockchain.
steemit already has "subforums" and things.
but i'm not really interested in pursuing this discussion further since in my opinion it's already a solved problem. no need in beating a dead horse...