The one part of the HUD I've always been disappointed in is the party frame. Coming over from WoW as a healer main used to using Grid, it continues to drive me nuts with the poor layout 6 years later.

I don't need to know how much mana any party member other than a healer has, buffs/debuffs could be made clearer (remove display of buffs/debuffs that have no impact on other party member choices, add display of duration remaining in the party frame rather than forcing party members to target the affected party member to get the duration). Tighten up the display with HP bar stacked on top of mana bar so the frame takes up less screen space.

Other improvements/options always welcome but updating the party frame would be far above the rest in importance to me.