-Delay Mexican independence through averting the Latin American Revolutionary wars. Mexico and the rest of Latin America continues its rapid, stable development throughout the 1800s until its far stronger than the fatherland. Additionally, this means a Mexico that has Central America, its north, potentially Louisiana, Cuba, Santo Domingo, Puerto Rico, the Philippines, and maybe (although its a stretch) bits of Venezuela.
Why would Mexico's population grow to the size of the US just because it remained part of Spain. Also, even if Spain kept Louisiana this doesn't mean the US won't eventually go to war with Spain to try to reach the Pacific. I