I'm not particularily great with history, but what interesting things can you give Mexico? I don't really know.
Personally I would love to see the Phillippines! Just because it brings attention to a region that rather lacks it, it in my opinion. And they would have tons of possibilities to interact with the new navy mechanics. They can have interesting decisions too. Support USA historically or try to anchor themselves with Japan and throw out the USA to help realize the Asian dream of owning the Pacific. Part of what made the Pacific Theatre so boring for me was that the navy combat just did not feel very engaging. This expansion can totally fix that problem. They might even serve as a vital role in the downfall of the US.
Personally I would love to see the Phillippines! Just because it brings attention to a region that rather lacks it, it in my opinion. And they would have tons of possibilities to interact with the new navy mechanics. They can have interesting decisions too. Support USA historically or try to anchor themselves with Japan and throw out the USA to help realize the Asian dream of owning the Pacific. Part of what made the Pacific Theatre so boring for me was that the navy combat just did not feel very engaging. This expansion can totally fix that problem. They might even serve as a vital role in the downfall of the US.