Some things to consider would be time zones and travel, existing rivalries such as they are, and whether the schedule of the other league matches up. Also worth asking- what exactly makes it worth it to both leagues, and would a lot of people be interested in watching it? You know- the usual questions that go into an exhibition friendly.
They should get rid of the All-Star game and just name the top 11 in the league. But if we had to play another league's All-Star's it should be Liga MX. We are most similar with them as far as talent, I would say a bit more talented no matter how the Concacaf Champions League keeps turning out. I would rank both leagues between 8-10 in the world
I do like a lot of things about the Liga MX idea (though I realize you're not that committed to it as a first choice) starting with the value of a geographic rivalry, and it's really the most important rivalry that North American soccer has. My one question would be how the scheduling would work out in a way that makes sense for both leagues. They're on very different schedules, what time of year would work for both of them?