On Alternate History wiki there is a timeline called 'Mexican Empire' or something like that. It is very ASB and to be honest, reads like something Turtledove wrote (seriously, the CSA acting and being defeated like Nazi Germany and then somehow becoming a superpower like the USSR?)
Either way, the premise, of Mexico remaining a monarchy and becoming a superpower is reasonably good in my opinion. So the challenge is, make this work in a non-ASB way.
The criteria for success are:
i) Mexico must retain it's 1846 borders.
ii) It must have a GDP that is comparable to the OTL USA (bearing in mind it will include a huge section of the south-west and west coast of the United States).
Apart from that, anything plausiable goes