Sorry if it's been suggested, but how about a bases mode?
Have different bases like the malibu, the mansion, the ice cream place, etc... and each character can take over bases, then you can go and take over other peoples bases etc. Say for example you have 3 pink ring thingies (like in the pizza place) that you stand in to take over the building
Sorry if I didn't explain it well, it's 5:30am... need sleep