Circling back to Texas and Mexico (I'll reply to the Deseret stuff in the linked thread above), I too like the idea of getting Germany and Spain involved.
Here's a couple of "what ifs" that came to my mind as I've been reading the responses.
Germany could be in it to work both sides of the border. They are currently (as of Zeppelins & Bombers) considering selling bombers to Texas to shore up their own inventory, an order that would make sense if another war with Mexico was brewing for Texas. They (Texas) have the smallest bomber force of any major nation in North America (cited in Z&B).
However, considering Germany's ties to Mexico, I could also see them selling Mexico the same thing. If this was me, I would probably throw German mercenaries into the concept, working for one or both sides, like the "Hessians" from the Revolutionary War. I could even see a major border conflict between Texas and Mexico echo the Spanish troubles in 1936, where larger, better equipped countries used their land to combat test their own weapons.
Spain's part to play could be to support Mexico, or to covertly try to undermine their Mexican allies with the idea of recolonizing the embattled country. I think imperial-minded nations are few in far between in CS due to isolationism and the current (1937) political landscapes across the world, so this might just be me thinking out loud. In my mind only Japan is actively trying to expand its borders and consume other nations. Most of the others are defending what they see as their borders and/or glorified saber rattling.
If WWII broke out in Mexico, I think Texas would not appreciate the endgame. I suppose it would depend on which nations threw in with which other nations, but either way, having a heavy military presence from multiple nations right on the Texas border would be more than they could handle, and both sides of the Rio Grande would suffer from the destruction.
All just my opinion, of course. I haven't thought this much about the CS geopolitical climate in more than a decade.