update: fep-9988 is on hold for now, as it is quickly morphing and evolving. due to nebulous boundaries of the concept of a "forum". it's seeming more like a rough and general conformance profile for general-purpose social networking, and the only real difference between current mastodon and this stuff is that mastodon doesn't have explicit conversations or threads.
so anyway: fep-8007 "A conformance profile for social networking services" is what i'm working on for now.