To give an actual example, implementing an entirely new game feature that does not currently exist in EQ2, but could piggyback on the Quest system (I think). Specifically, the Vanguard:Saga of Heroes "Diplomacy" sphere. I think it really was just questing, or even "crafting" in a distorted way, where you're making progress towards an end goal.
The UI question comes from the "cards" that would have to be implemented in the UI, and the packets that would have to tell the UI which cards were randomly drawn by the opponent, and of course handling all the progress stuff.
I am aware that we cannot teach the client to use new structs, but using something that exists but maybe currently isn't used by the client (lots of unused elements, I am told) maybe it's possible. I do imagine it would be a ton of work, though.
But the first idea that lept to mind was a UI to manage the Bots, akin to the Pets UI, but more detailed since they are more or less, group members.
But anyway, time for me to slip back into retirement

My work here is done.