More homies with their rescue missions, loyalty missions and more story around it. If you bring back Viola, Oleg, Josh, Zimos and add Jane Valderama as homies and they all have their nightmare missions, loyalty missions and perhaps some secondary quests, you add about 5 to 6 hours to the game easily.
A new game+ option where you can play with your fully trained character, having another shot at challenges you missed.
An Insane difficulty setting wich adds more challange. Because.. the names of the difficulties are all wrong. Easy should be very easy, normal should be easy, hardcore should be normal, and then you should get hard and then very hard. I know there are mods for that and I already downloaded it, but still, I would like it if Volition themselves added difficulty to it. And not just give the enemies more health and stuff, but also give more enemies. Especially with co-op.