The next release of the blockchain will support everything necessary to implement a forum at the blockchain level.
It will just cost you $0.02 to post
I would imagine a per byte (per character) fee on top of a small fixed fee would make more sense. Then again, having only fixed fee would filter out the more trivial posts, but there is a risk of people abusing it to host data for cheap.
Moderation would be interesting to implement. Can posts ever be deleted to save space? Can they only be marked as removed by moderators so that user clients simply filter out those posts (assuming they trust that moderator)?
I wonder what the ideal fee would be. If people posted just as often as they do now even with a $0.02 fee per post, which is highly unlikely, that would be a revenue of approximately $240/month to run the DAC and the forum hosting. If we ignoring the costs of paying the delegates a basic block producing salary, then that budget would be sufficient if it was just paying the server costs of a single provider. But decentralization means it needs to be distributed to many different hosting providers. If you even have 10 different hosting providers (perhaps the delegates) to provided decentralization, the budget has already been knocked down to $24/month for each provider. Add in the fact that the average posts per day will drop because of the 2 cent fee and it doesn't look good.
In my opinion there has to be a different incentive structure to provide decentralization. There must be some profit incentive to motivate many people to backup the data and have it available in case of emergency situations (for a price). But under normal operation, I think it would be fine to just trust a single provider (or maybe two) to host the public forum data. Then there is the question of how to charge for the service. Is it a one time fixed fee per post? Is it a one time variable (based on size) fee per post? Is it a reoccurring variable fee to keep the post data available or else it is freed to save space? In which case perhaps the person who wants the data back needs to either have a local saved copy or make a bid on an open market for someone who has a copy in long-term storage to get it out and repost it into the decentralized cloud to claim the reward. Do the post readers need to also pay a fee to cover the bandwidth costs of the host delivering the post contents to them?
This is not a trivial DAC to build. I think the first step is rebuilding a centralized forum to follow some standard protocol so that a local client can locally reconstruct any part of the forum by simply requesting the content-addressable blobs of data from the host (or anyone else on the internet) that are necessary to reconstruct that part of the forum. I then think a Maidsafe/storj like system is necessary for hosting and providing these blobs of data (with the right economic incentives to make it sustainable and difficult to abuse). Then, a market for hosting and search indexing providers could open up who use these tools as the foundation to provide forum access to users. The user's local client would still follow the same standard protocol so that their posts are accessible to other users even if they use a different host/search provider. The host/search providers can compete on their fees and would likely be paid through some micropayment channel set up between the user and the provider.