I could definitely see a "Urban" expansion with a map just entirely metro based roads.
PG have been pretty clear they'll never do that for a full FH game, but as an expansion I could see it working, just like PG will never do a full FH Hot Wheels game, but as a standalone DLC, it's doable.
The progression in it could be very NFS style with you starting off in a slower tuner car (Mazda 323 that's been on the leaked car list perhaps? Honda Civic EG return?) and working your way up the classes. Again something they will likely stay away from in the main game.
They could introduce new upgrades and popular upgrade features in something like this as well. Air suspension, New Forza Aero, Tire Lettering, Paintable windowshields etc.
I guess this sort of expansion doesn't really tie into Mexico very well. But then again, did Blizzard Mountain make a lot of sense in Australia? (Yes.. I know it snows there. But let's be real. Is Aussie known for its snow?) Did Fortune Island make a lot of sense in the UK? Certainly moreso than Blizzard Mountain but if PG were going for something that made perfect sense based on the location of the mainland, the Isle of Mann would be the logical choice.
What has me more curious is the Project Uspide Down. Clearly a Stranger things reference, but these codenames seem so ambigious it could be a reference to a lot of different things.