A couple years back I suggested that they create a full trilogy of games inside the first they create, so that in case for sequels they'd already have a basic infrastructure in place to create content and additions in. The city they have, doesn't need to be rebuilt. Just fixed and filled. And graphics certainly don't need to get better.
Eeach new campaign startable from the first menu, each adding its own set of new character lifepaths and storylines (possibly even slight interconnections between the campaigns), and also general additions like skills, perks, interactivity, sidemissions, etc, that every campaign can utilize, and each improving the city in it's own way.
So that in the end they'd have one game with three campaigns. And if it's all successful, why not add a sort "freeplay" mode where there is no set storyline (perhaps just one that sets you out to make your fortune which you can end in various ways as you please with narrative slides according to what you've accomplished and how, and how things went on after that -- or simply keep playing if you wish), but simply a huge city full of different things to do, different characterbuilds to build, interactions to have and sights to see.