Is my presumption that such a distinct variant will emerge sound, do you think?

Whenever you split off one group of speakers from another, you can assume the language will become different over time. There are already regional differences between various New World Spanishes, Argentine Spanish being one of the more extreme. As the population of hispanophones here in the States gets refreshed from time to time, I'd imagine that the language of their children would assimilate to the US variety.

Obviously I was thinking of the influence of Spanglish, but I also wondered if a "homogenising" of sorts might occur with the different variants all thrown in together.

I have seen that kind of homogenizing effect in my hometown, where Italians with different regional dialects accomodated one another, and incorporated English words. I used to be amused that my grandmother would speak her heavily dialectal Italian (Genoese) with migrant Mexican workers who rsponded in Spanish, but some kind of communication was taking place.

Ceci n'est pas un seing.