Destiny. The setting's quite interesting, and the enemy factions are fascinating. It's a goddamn shame all of the actual story and lore was stuffed into cards I have to read on the internet, instead of being put into the game one way or the other.
I wanna know what the huge threat back at the Cabal homeworld is.
I wanna delve deeper into the struggle for survival the Eliksni suffer on a daily basis, I want to know more about their history and what they were like before the became 'the Fallen'. I wanna how their hierarchy, civilization and daily lives in general worked before shit hit the fan.
I want to see what the Vex are truly capable of with their powers and unified hivemind-like thoughts. The game does no justice to them in that regard.
I wanna see what the process of a Wizard draining light from a Guardian is like.
I want to know more about Rasputin, how he thinks, how he operates, etc.
There's so much shit to explore and expand upon, and Bungie isn't doing anything with it. It frustrates me, because there's just so much unused potential!
Universes that you'd like to explore deeper than the games do?