This is how I see something like this. Of course, I'm not in charge of anything more important than making sure oil gets changed or grass gets cut.
Samsung can make such a phone. The technology side is easy, and already figured out. There would be lapses of service when switching from one carrier signal to the other, and your battery would die very fast looking for signal across 14 different LTE bands on 14 different frequencies, but it can be done using existing tech. New tech possibly alleviates some of the issue.
The FCC can't refuse to approve a device that follows regulations and can safely connect to existing radio transmission signals. Staying under the SAR rating may be difficult, but that can be mitigated. Again, new tech can help here.
Carriers can stop a device like this from connecting, but they won't. As long as it meets any approval they have garnered through the FCC, it would be difficult to justify. But they don't have to sell or finance anything they don't want to sell or finance. They also don't have to offer any special services outside of the minimum required from any agreements they made to use/lease spectrum. No HD voice. No VVM. No location services from Verizon (other than raw GPS). None of the new "free" data for streaming. No Verizon or AT&T messaging. No access to dedicated MMS gateways. No tethering through carrier gateways using carrier DNS. You would get a basic LTE signal to use for voice or data. Again — these issues can all be sorted out using existing software on existing hardware, and new tech can make it work easier/better.
And you'll be charged out the rectum from all 76 local and nationwide carriers through some sort of aggregated service agreement from a company that represents them all. Existing roaming agreements that fall outside of any FCC mandated spectrum regulations would not apply to something like this. Pine Belt Wireless and Mid River Communications are companies that you may have never heard of, but they exist, they own their own infrastructure and you can bet your booty they aren't about to let you roam on it the same way they let AT&T customers roam when in areas where they have "exclusive" coverage agreements. They gots to get paid, too. Only paying customers get to use their network. I live in such a place. Verizon offers no service to my zip code, because US Cellular "owns" that wireless spectrum here. But if you come visit — call ahead, I'll roast a pig — you get some of the best Verizon service you'll ever find because they pay USCC for "free" customer roaming, and all brand new equipment plus very few users makes for some really great connectivity. Because I live here, I can't (legally) do the same and have to go to the fancy new US Cellular office in town and sign up. If I use a Verizon phone too often here, Verizon shuts off the service because it violates their terms and my sister gets pretty pissed because she has a huge termination fee to pay. Sprint Pre users who used the roaming trick to get Verizon service at home know what I'm talking about. Also why I no longer have a Sprint account, but that's another story for another time
That's the one thing they can and will do — make you pay dearly for this type of access. Nobody can invent new technology that changes this. New laws could change it, but when the people making all the money have all the influence they can afford to pay for (hello AT&T and Verizon, never mind Comcast and GE and Boeing and everyone else with enough money to buy politicians hiding in the back wringing their hands in anticipation) it won't.
That doesn't mean it cant or wont happen. There are plenty of people — both private and corporate — that would find this sort of system worth the $1,000 per month (speculative) fee. But I doubt there are enough to convince any company to make the first step.