What if the machinations undertaken during/after the Napoleonic Wars to keep the Spanish possessions in the orbit of royal/imperial Brazil had succeeded?
Well, even if it 'd do require a different development and/or consequences of French invasion and Peninsular war, but to answer based in the OTL, the reaction of the local Spanish and Criollo officials and/or elites to the Braganza envoys proposal, if they 'd be anyhow successful...
Then I'd think that the more probably consequence aside of a division among those loyal to whatever 'd be their new Queen and the rest that 'd be supporting and starting a lot of rebellions and giving aide and support for whatever rival/alternates royal candidates they 'd be found.
What if more countries experiment with postcolonial monarchies?
What if Mexico's first postcolonial emperor is the scion of some European house?
Well, it 'd depends on a different development of the Napoleonic/Peninsular wars and/or from different or not the Revolutionaries wars... But, in the case of the Spanish-Americans, I would guess that 'd be a lot of political solidarity and intermarrying between them and probably would have conserved/continued mostly intact the colonial era socio-political structures. Thus, I'd guess that mirroring the OTL Brazilian one that they 'd be facing often political uprising attempts and/or rebellions...
Finally, for Mexico, the only possible alternative to OTL, that I can think about 'd be crowning to a Bourbon prince from the Spanish royal family... Also, about the consequences, first and foremost 'd be the legitimacy and prestige that a Spanish Bourbon 'd have for the whole population, while for the local elites, it 'd have meant the guarantee of the colonial era socio-political continuity, but now with them in the upper strata of the Mexican society while avoiding to cut all the political links to Spain.
Also, in this scenario, 'd be possible that these same factor 'd have allowed that if not ruled directly from Mexico that the OTL Central American nations, 'd have been possible that they 'd keep their unity under a monarch of their choice among the Spanish royal family...