An idea: how would you go about to get the Galvez proposal for the Bourbon reforms adopted as is; that is, New Spain also gets split up in two viceroyalties, with a viceroyalty of Nueva Vizcaya covering Coahuilha, Tejas (minus lands conquered on Tamaulipas), the Californias, Sonora & Sinaloa, Nuevo Mexico and Nueva Vizcaya. IOTL IIRC the main problem they had was their extremely low population (a problem, that Rio de la Plata had to a lesser extent, however, since it only got its first viceroy decades after the Bourbon reforms) - also apparently wiki says something about them costing more in military budget than their entire government income (which I didn't know...).
Would this be likely to lead to another republic in the Southwestern US?