Underwater anything. That’s my pick.
Ultima Online didn’t have swimming, just boats, but Classic EverQuest rather impressively had full-scale underwater zones and underwater combat from the beginning. And it was pretty messy! Ask any EQ1 vet about corpse runs in Kedge Keep and she will shudder, guaranteed. But that ancient game had it nevertheless.
So it’s weird to me that it’s 2023 and we still have MMOs that don’t have swimming or underwater content or all, and the ones that do, well, it’s a little better than what we had in 1999, but not nearly as much as you’d expect, given how much everything else has improved. Water is hard!
What else are we neglecting here in MMO design land? What MMO feature hasn’t been developed as well as it probably should’ve since the dawn of the genre?