A Waterdeep expansion executed well would be the greatest expansion!
Water deep fits the original DDO feel and I think it would be fun and hit. But I can’t see how epic would fit. Although they did implement epic dinosaurs pretty well. The only other concern is Waterdeep would not lead to any new race or class introduction.
My ultimate vote goes for a new real class and/or race. The archetypes really aren’t a hit. In my opinion most of them are just another past life to get. Tabaxi was a great race addition. Warlock/Alchemist were great class additions. I would love to see more prestige enhancements. Maybe even add some that require multiclassing options before they become available. That would be really fun.
Cavalier was mentioned as a class, so let’s use this as an example. Cavalier trees unlock once you acquire 3 levels of fighter and 6 levels of paladin. Now you have access to 3 new enhancement trees that ultimately are more powerful than the basic trees from fighter and paladin. At least enough to compensate for the loss of going pure.
Some other options….Samurai would require 6 levels of fighter, 3 levels of cleric and 3 levels of monk. Dark Knight would require 6 levels of pally and 6 levels of cleric. Trickster would require 3 levels of fighter and 3 levels of sorc/wizzy. Sage requires 3 levels of wizard and 3 levels of cleric.
To me this is how archetypes should have been done rather than offer as a whole new class option. New past lives are not introduced, but essentially creates a new way to play.