I like the US world idea. I've seen people play around with airlines bound to certain states before (especially Texas and California), but they usually open a hub in a major airport elsewhere when they grow too much. There's certainly more than enough demand to make this very playable and fun.
Here are few ideas and potential conflicts I see:
I know you probably just mean the literal 50 official states, but I think it would be more interesting to include the territories of the US as well.
So would you think including Puerto Rico, Guam, Northern Mariana Islands, American Samoa, and the pacific protectorate states like the Marshall Islands and the Federated States of Micronesia be a valuable addition/inclusion? ... and would they be all individual states? Or would you lump Guam and NMI together? Maybe Guam plus NMI plus pacific protectorates? You could also maybe add American Samoa to Hawaii because of its super low demand?
Also, some states may be a bit stronger than expected due to the actual locations of some airports. For example, Cincinnati CVG is actually in Kentucky, and both Dulles IAD and Reagan DCA are in Virginia. What I mean is, these airports are generating demand for a city in a different state. Newark EWR is in the same situation. I guess one could argue that because it is operated by the New York Port Authority it could be lumped into New York as a "country" in this scenario. Because CVG is meant to serve southern Ohio it could arguably be lumped into Ohio as a "country". Then there could be the issue of making Washington D.C. a state to lump IAD and DCA into it, just to add more variety I guess.
Then again, you could just leave all airports in the states they are currently in, and make Puerto Rico, Guam/Northern Mariana, and American Samoa into states. This would probably be the easiest and fairest outcome. Only downside is that American Samoa would be quite difficult to operate profitably in.