A year ago I'd have supposed that Google would lend the branding to anyone who wanted to work with them to distribute good AOSP patches/overlays and images available to download. Now, though, they've tied it pretty well to "Google Play" in their commercials and branding such that I expect them to hold onto that branding for themselves.

I kinda think everyone is reading way too much into this "X" branding. It's a codename. I'd bet some spare change in my pocket that it will be a Nexus-branded device.