I think one of the best ways to do this is to make users, especially new ones, understand that their forum account is actually like an investment in the future. Not only because of the possibility of participating in signature campaigns or bounties, but actively participating in the forum is the best way to learn, since that is what many new users ask, learn about trading, which are the best exchanges...
And although the forum is "more focused" on altcoins, that does not mean that we should talk less about Bitcoin, on the contrary, talking about the entire cryptocurrency ecosystem in general is what can generate more conversations and better threads/posts.