We really have 2 options at this point...the Lumia 635, the Sprint version, maybe that can go on the Insider Program and be upgraded.
The 2nd more wild option is the 8XT. MS has the stock image now, and it can be recovered using the Windows Device Recovery Tool...so they do have the good image from HTC for the 8XT. Having that would make it possible to release WP10 through the insider program for it. Using it on one of our 8XT's actually fixed an issue Sprint was clueless about, so the Recovery Tool most definitely works.
I still think, that unless Sprint is working with HTC, LG or some other 3rd party, we're out of luck on the new Lumia's and WP altogether...and I'm bummed about it.
My friend just gotten the Lumia 950 and I want to say WOW! that phone is a beast. It just one awesome hardware. It so fast and love the camera so blows away iPhone 6S camera. If Sprint ever brought the Lumia 950 I have feeling it will get sold out quick just like it was sold out quick on ATT