Wonder what government system it would have, I guess we'd probably stick with the US system and end up with the US portion dominating the rest of the country. One good thing, you can send all the wannabe soldier cops down to Mexico to fight the cartels
Considering a violent unification rather than willing annexation, is more likely that Mexico will be militarily occupied and Cartels will be dealt with either through negotiations to help stabilize the area or through force (probably and ideally a combo of the two)
That said, the US conquering Mexico would be disastrous for the US economy.