ToME: the Tales of Maj'Eyal

A dlc with waterbased races, a new starting location like the yeeks' but most of the content is underwater. could have a new cavebased area. No waterbubbles underwater, but certain parts of the cave are above waterlevel and you have to swim quickly to the next area.
Would be perfect for Yaech and Naloren.

As for classes, a Yaech psionic class or a Naloren exotic weapons focussed class would be fun additions.