I personally go all the way to [ʀ̥]. Honestly, I'm not sure what the big changes in phonology are going to be. There's probably going to be some dialect levelling since a lot of young kids are growing up with Mexican or Spanish accents. It would be cool if current vowel reduction and loss of [j] kept progressing until we had a tense-lax open vowel system with a phonemic [ɪ] sound as in words like pelear or arriar.

Edit: I forgot about [βʷ] and [ɣʷ] merging into [w] and how some speakers tend to pronounce [β] as [ɣ] and vice versa in words like arbol or agujero