Reddit has subreddits. What does Lemmy go with?
My personal vote is for lemmings!
Edit: I am personally leaning towards Sublemmy now. It retains the context of being a forum under the general sphere of Lemmy and the connection to Reddit lets people know immediately what Lemmy is about. Thanks to @BurningnnTree@lemmy.one for the comment!
That works okay as a shortcut since many are used to the term already, plus to get a regular feed you have to subscribe. Like any language, what gets used the most after a while will become the normal term.