ORD and SFO already have separate international terminals, but they're often not used for international departures by USA carriers. The same is or was true for BOS, although some airlines didn't use the international terminal for their international flights. LAX is similar, with TBIT being used by (not all) foreign carriers and having no domestic flights. MSP Terminal 2 was international (arrivals) when it was a cross between a hangar and a shed. Similarly, JFK T4 was once international, as is T1 now. [In fact, ages ago TWA had separate international and domestic terminals at JFK.] Part of PHL A is international. ATL E/F is mostly international but there are exceptions. So some airports are designed in a way that it would be easier to do this.

MIA and IAH, plus maybe ATL, are probably the airports with the most natural international connecting to international routes, primarily for traffic between Europe and Latin America. On the TPAC side, LAX would be the obvious location, although there's traffic between Asia and Latin America through SFO and SEA too.

Another idea would be to focus on USA-Canada and vv transborder connections, thereby increasing airline competition in North America. USA passengers would find it easier to fly AC longhaul while Canadian residents could use USA legacy carriers with fewer formalities. However, these changes would be more difficult to implement due to the current pre clearance system.