That's a great idea, except I think the rest of the world is going to assume that the big bad USA is trying to take over the world via "annexing into statehood" and will no doubt launch a campaign to stop us.
Mexico would be very difficult. Know the drug problems it has? We annex it, we inherit it's drug problems. Most people don't want to have to deal with any of that, thinking that we can just save money and energy just fortifiying our borders. Plus, the Mexican police are corrupted. You think they're going to be willing to submit to a new authoirty? No way. I forsee a second Mexican-American war if we annex this.
Now, Puerto Rico and the Virgin Islands? Sure, let's bring them in. I think we can live with 52 states.

I mean, we survived with just 13 states and last I checked, 13 was an unlucky number.