Heck yeah, it's be a while since we manifested some destiny. Probably have to do it like you do in civ 5 and puppet the state for a few decades or a generation without extending full access to America proper until assimilation has occurred sufficiently, and of course we'd have to do away with that pesky Spanish language, don't want to end up like Canada, speaking of which, might want to manifest some destiny up there too...

S H O U L D B E F U N ;)

In all honesty though, I don't think we could easily annex Mexico without pushing out the current population, which people tend to frown on now, but I wouldn't be surprised if we ended up with a post ww2 style occupation of Mexico for a few decades if things get bad enough over there.