Everything except sovereignty, and some of them are pretty good at working around that.
I'm actually sort of surprised that a company hasn't just "bought out" one or more of the smaller countries -- paying off the citizens to renounce their citizenship, say. It would be easier if you bought two, so as to avoid creating a bunch of stateless people -- pay Country A to grant citizenship to everyone in Country B, and pay the citizens of Country B to renounce their Country B citizenship. You now have Country B to run as you please.
I'm actually sort of surprised that a company hasn't just "bought out" one or more of the smaller countries -- paying off the citizens to renounce their citizenship, say. It would be easier if you bought two, so as to avoid creating a bunch of stateless people -- pay Country A to grant citizenship to everyone in Country B, and pay the citizens of Country B to renounce their Country B citizenship. You now have Country B to run as you please.