I would also suggest sending Brazil to South America
Some possibilities :
Make Argentina own Paraguay, Uruguay and Chile, Brazil with its historical territories, USA smaller than in OTL at 1836 (mostly 13 colonies area, with some neighbours to the west, it would still have cores but would not be way too strong at start), and then Mexico with...i don't know.
Anyway, La Plata should be an interesting possibility of a power, but USA, Mexico and Brazil should be really powerful.
Oh, don't forget the governments

i'd bet that you had a great deal of influence at their formations, so...Brazil and the USA being monarchies? While Mexico would be a dictatorship and La Plata...something that should be intersting.
USA should have a strong, professional army and a well-distributed and literate population and strong industry, Mexico being huge but with its population being concentrated on some regions and weak industry.
Brazil would be big, have a powerful navy, african colonies and a kinda big but mostly illiterate population (they revolted in Africa, right?) with a developing industrial capacity
La Plata could be big, but small population (tough it would be literate and with their main culture), they would have a sizeable navy (not as big as Brazil's), good ammount of reforms among other things.
So in such a setup it would be :
Best Navy : Brazil (closely followed by USA)
Best Industry : USA
Biggest population : Mexico
Best Army : USA
Biggest Army : Mexico
Biggest % of literacy : La Plata
Biggest industrial potential : 1. Brazil, if it gets to raise literacy ; 2. USA, if it conquer its cores ; 3. La Plata, if it's population grows fast enough
That is just an example where your emperor abandoned teh americas or just managed to get kicked out of it
Really, lots of possibilities, maybe even add a Gran Colombia in...the Americas could get really interesting in that game considering how different from OTL it was like

IMO Brazil and the USA would be perfect American powers, with Mexico and La Plata often threaning their positions with a strong army.