What I did in this case was to simply have a half dozen non-player characters and groups who could potentially turn into BBEGs, depending on whether the PCs run up against them. I worked out what their various agendas were and how they intersect with each other. And just set the PCs free to follow their own aims. Sure enough, they've managed to piss off three groups, make a couple more indebted or at least interested in them, and given me fodder for dozens of sessions worth. And one of the big advantages, IMO, is that the players feel rightly that they are the ones making their own story and decisions rather than just creating a story which I had prepared.