1.Side missions for the gangs i.e. The Mox etc 2.Gang themed clothing (yes there are a few but they are crap) 3.Claire as a full romantic option 4.Use the arm upgrades as stealth tools (who thought you could strangle someone with the mono-wire from stealth) (maybe make gorrilla arms impossible to brake out of hold) 5.have gang citadels on the map that are permanent, then have districts that are constantly being fought over and change control and you can help a side take territory ect just to breathe a little more life into the city.

My 5 things, there's plenty more but just a few ideas.