With some of the new-generation chipsets from Qualcomm and the like, it now becomes possible to build a truly universal world phone. CDMA here, GSM over there. LTE everywhere. The only restriction, I suppose, is how many different antennae (and the associated op-amps) that you want to cram into the case.

That make it at least hypothetically possible for an MVNO to allow for roaming across all four US carriers, or for the carriers to roam across all of their peers. Today, MVNOs seem to be targeting the low-end price-sensitive market, rather than the "damnit, I want high quality service everywhere" market, which currently seems stuck with Verizon.

We'll see.