If it's actually 10+ players, that's the point at which you can make substantial progress in Wurm. Although there's a lot to be said for getting Unlimited and bumping rates anyway, just because a private server feels fresher.
And ah, fuck it, since I'm here, obligatory Minecraft.
I'm interested in a space MMO but I don't have a bead on good ones. Starport GE was my goto but it's broken on wine.
I am vaguely interested in giving Ryzom another go and not screwing up my character build this time. But I don't recall Ryzom being particularly amazing, it was just mechanically interesting.
Could dick about in the Mana World I guess, or some other pixelly thing. Though I don't recall any notable names beyond that, and honestly, it's kinda low-tier.
Oh! Puzzle Pirates! It's legitimately a really good MMO! Like, the traits that people often desire in MMOs are all present in it. So yeah, it's a kids' game. But it's also fun for adults. Especially once you start reading up about sword fighting pro play.
Age of Wushu has a lot to it, plenty to explore and enjoy (and the combat is loosely fighting game-based, in a way that makes sense in an MMO), but I can't actually in good conscience recommend playing it, because the devs are incredibly greedy. :/
ED: Mabinogi! Which is kinda cool across the board.
[Salty that Elyria isn't out yet. And Gekkeiju. And......]