A name for forum- and aggregator-style fediverse software. and instances sounds like a good idea to me. I agree with the points you and @ada@lemmy.blahaj.zone make elsewhere in the thread about the problem that most people coming from Twitter currently equate Mastodon with the "fediverse" as equivalent to Mastodon (a problem in general because it leads to centralization and marginalizing other implementations, and an even bigger problem currently because of Mastodon's reputation for anti-blackness and reply-guyism), and not wanting to have similar dynamics with people coming from reddit.
I'll have to think more about the specific term threadiverse. I see what you're getting at but Mastodon / Pleroma / CalcKey etc all have threads as well even from a microblogging perspective, and Kbin also has a microblog (as opposed to forum) view.