(please correct me if I've gotten the facts wrong...)
I'm sure Spain wants nothing to do with the proposal - but someone else (another former Spanish colony?) might find the idea an ideal means to tweak Uncle Sam.
The UN, of course, would be all too eager to spend buckets of money (mostly ours) investigating how we've gone wrong in upholding the treaty.
And, although every Puerto Rican I've even known was just tickled with their unique status, there is no telling what the UN might have to say about the proper course for an ethnic enclave off the coast and 'suffering under the yoke of 'gringo domination' (or something).
On the bright side, we could condemn the old building in Turtle Bay, pull diplomatic immunity from the UN, hand over a lot of newly empty hotel space in Puerto Rico, and perhaps let 'em both go together.