As I've mentioned before, really hoping that mods are part of the "things we've never done before". With the new ground Bethesda's made in bringing mod-support to consoles, and MS merging their Xbox and PC stores, an "Xbox Mod Workshop" would be incredible, and something that unlike their other two big competitors, they'd be well-situated to pull-off. It would also ameliorate many concerns people have with both the Windows Store, and the Xbox-PC unification. If bringing Xbox games to PC meant mods on home-consoles, I think you'd find audiences supporting universal Xbox games far more.
Just think of the possibilities:
Scalebound with effectively infinite custom quests, new areas, weapons, dragons - hell, hate the protag? Replace him.
Dead Rising 4 with a constant influx of bizarre weapons, vehicles, playspaces and zombies.
Perhaps Sea of Thieves and future Gears games too. MS have a ton of games running on Unreal Engine 4. Implement it into your UE4 Xbox dev tools.
Would be so good.