I'm gonna break the rules and list things I'd add, not just one.

Butt/pec slider for male PCs
Instanced housing for everyone where you have to start out with a small and can build up to a large.
A glamour system that's a mix of WoW, GW2 (including the dye system), and I've heard ESO's is good to so that one as well (I played ESO but idr the glamour system there).
Some sort of incentive to draw people into non-venue RP. Please guys, RP isn't just dancing and emote spamming in a venue/saying a couple of lines in character and then getting paid gil for it at the end of the night.