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I think that the main problem is that the forum must be available to everyone and easy to use for beginners. If the group is closed or Nodes could have a firewall, this will reduce the features of the forum.
How large is the blockchain that the client needs, and how can shares be modified, deleted, does it require approval from all nodes, or can the user change it? Since there is no encryption/decryption, losing the private key or having another party know it means you need to create a new account, or is there a way to solve this problem?
The main idea for such decentralized forum, came from here which i'm going to describe. I think it will clear things a little.
Imagine this very centralized forum (bitcointalk). every post i do send, a mysql query will be generate like this pseudo code:
insert into Forum_Posts (user, text, date, ...) values ( 3311025, 'the post context', date.now()) -- note that my userid on bitcointalk is 3311025
My main idea was to convert these queries into blocks, and form a blockchain with them.
Every new user first need to download the blockchain, apply it to a local DB and it have a readonly forum on localhost.
Nodes can have or have not firewall,
About pvt key, losing or exposing of it will be same as losing pvt key of a wallet in bare bitcoin network. all funds will be lost/taken.
About simplicity of use, i think it will be available as software like bitcoin core.