The biggest problem with Mexico is the drug cartels. There are other problems but drugs are the biggest. Deal with the drug problem and half the problem will go away. How to deal with it is another question, however. Legalize it, clamp down, treat it like a military conflict, come up with an invasive GMO crop that has some additional benefit like a nutritious food product or fuel source while it kills off coca or makes it ineffective, who knows?
Over the long term I wouldn't be opposed to Mexico, Canada and the Caribbean eventually being annexed into the US. The key phrase is "over the long term" that will allow plenty of time for those areas to gradually become "Americanized," which is already happening on its own especially culturally. The Bahamas has linked its currency value to the US dollar; a 1 to 1 exchange rate. All of the Caribbean watches American TV and movies and most of their more popular radio stations play American music. In Mexico, students are required to take English classes every single year and as a result almost everyone is bi-lingual. What I've learned is the illegal Mexican immigrants we see in America are not average Mexicans for the most part but are very low economic and social ladder even there comparable to our homeless and welfare class. When I was in Mexico this year most of the cars I saw on the road were newer than mine. We got to drive through a residential neighborhood and it looked like a typical middle-class American neighborhood. We saw a Sam's Club and a grocery store and it looked like we could have been in Louisiana or Georgia.