Flarum has been there for awhile, their previous product was esoTalk.
PlushForum appears to be new, but not sure how great their software is since it's pay-to-use (and hosting from them not self hosted).
I hadn't heard of either of them until you mentioned them but having had a look at them they both look interesting, Flarum looks the most developer friendly version of the two. It should be interesting to see how they develop and if they become big players.
I've heard of Gosora, it's made by this mysterious person called Azareal, I don't know who that could possibly be, but there's always the chance that they're in your midst right now 😱
PlushForums, if I recall, is more of a SaaS player than a software, so it likely wouldn't shake up the line-ups too much. Flarum seems to be positioning itself as a better Discourse, although I personally don't like that sort of style.