HND is interesting - it's great for O&D and domestic connections, but East / Southeast Asia connections are difficult because the onward connecting flights are also similarly slot-restrained and more premium. When JL/NH/CX can regularly sell HND-HKG for $800 roundtrip, why would they want to open up the inventory for spillover US-HND capacity to facilitate connections?

At the moment, HND is VERY well served by all major US hubs, and I struggle to find a place where AA could profitably add a flight. AA's original proposal was LAS-HND, but LAS being a leisure destination and low AA connectivity makes it hard to work. SEA is already served by NH and DL. MIA is somewhere AA's 789 could fly to for sure, but it will be heavily reliant on Latin American connections from Japan, which are probably not the highest-yielding traffic.

If they had to find a hub, I think PHX is a better choice. With AA retreating in LAX, PHX would enable more connections in the Midwest / Mountain Time Zone Area without a backtrack from DFW. And given the mess in JFK nowadays, it might even be an appealing alternative to major East Coast cities like RDU/CLT/PHL/DCA/MIA. If it can support a LHR flight (in addition a BA PHX-LHR), it should be able to support a HND flight.

In the end, AA/UA/DL should come to the reality that no one is going to fly them to premium cities where JL/NH are alternatives. The advantage of US3 is they are able to facilitate connections to deeper parts of the US, and should plan their flights accordingly.