AA's DFW hub, which has 3 flights a day to Tokyo, already has great connectivity to the most in-demand markets from Japan and Asia - namely, Lima, Mexico City, Santiago, Buenos Aires, and Sao Paulo. European carriers, and Emirates and Qatar Airways, also provide excellent connectivity to Brazil and Argentina.
...just let me finish your statement: "...for the US".
Miami might be a gateway for Latin America for the US. Period. Not for the rest of the world.
But for Japan (and north east Asia), MIA is one of the best airport which has almost the shortest travel time as well as good connectivity.
Of course MEX has good connectivity too but AM is in ST but not OW
For cities in Pacific side of S. America, LAX or DFW are actually less of a detour than MIA. Japan-EZE is also quicker through DFW/LAX. Meanwhile, the largest S. America market from Japan, Brazil, is quickest through JFK (In fact, NRT-GRU great circle route fly right over NYC).
As for AM - they actually codeshare with JL, although I believe only on NRT-MEX flight and Mexico domestic flights. The traffic between Japan and Mexico is mostly automotive industry base anyway.
For the flight itself - we'll see. MIA has been trying for years to get a TPAC flight (Mainly JL), but JL so far is not biting. It's still a very long route that would required some fairly premium yield to work. Japan-S. America traffic in general are mostly VFR (to Peru or Brazil) and tourism. Japanese business are not really heavily present in S. Florida in general (You got the Sony Latin America HQ, but that's about it. O&D demand would be mostly tourism and limited low-yield VFR for those Filipino Cruise Ship workers.
Yes, if there's any carrier that can possibly make a TPAC to/from MIA work, it'll be JL. CX is the next candidate, but that's a even longer flight with even more limited demand.