A full frame body that was somehow close in size to the NEX-7 would be lovely, but that's magical thinking for now, and for the foreseeable future. The NEX has a registration distance of 18mm, vs. 27.8mm for Leica M. It's clear by now that getting adequate performance from a typical (symmetrical, more or less) rangefinder lens on a crop frame is a challenge. Only the NEX-5n really accomplished that, and there are probably multiple interlocking issues that make it a challenge. Though it was initially criticized as a failure for rangefinder users, the much larger issues with the Fuji X-Pro1 make the NEX-7 look like an engineering miracle.
Now imagine scaling up to a full frame -- how would even the best current Sony sensor look with compact lenses designed for a 27.8mm registration distance, if it were doubled to cover the full 35mm frame? Ugly. Leica so far has the only 24x36mm sensor for short flange lenses, and it requires sometimes extensive corrections that are dialed in for each specific lens. Hardly a universal, interchangeable system.
What does this mean?
To me, it means that at minimum, any Sony full frame mirrorless will have a flange focal length of at least that of the M system, in order to even have a ghost of a chance of decent edge performance -- and that's only with lenses specifically designed for the camera, and probably carefully profiled and corrected in software. That means a body that's at least 10mm deeper than the current NEX bodies. It also means that Leica users will lose that precious 10mm that allows room for a non-optical mount adapter. Honestly, if Sony were to develop a full frame mirrorless, I wouldn't be surprised if they pushed the registration distance even farther, to avoid the kinds of problems that seem inherent with the M system. Sony might conclude that something more like a 30-35mm mount would allow them to design compact wides that don't require software correction. And why should they try to accommodate Leica shooters, anyway? Sony, just like every other major camera manufacturer, is interested in selling lenses, and not just bodies. Why accommodate users who want to use your body to support another company's lenses, when you could steer them toward some excellent lenses of your own, that perform better on a dedicated platform?
For Leica shooters looking for an alternate full-frame platform, it seems the only likelihood in the medium-to-short term is a third-party native M-mount camera. The only possibility I see is Cosina, though I'm not terribly optimistic there, either. The Cosina/Voigtlander/Epson body was just a repurposed Cosina, and far from an engineering marvel. They did the much nicer Zeiss Ikon, so maybe that could be a base platform for a full-frame digital -- but once again, they're still faced with solving the lens problems that Leica hasn't exactly put to bed.
(edited to add, in response to Ken's post: a native M-mount body with an EVF and handling like that of the NEX-7 would indeed be appealing. I don't see Sony doing it, though, since they'd want to steer users toward their own lenses. Cosina could, of course, but they're pretty conservative with design issues, essentially just building very conventional rangefinder cameras. They don't have the digital engineering resources of Sony or Fuji, so they'd have to license something or fairly aggressively recalibrate their camera development operation. It's hard to see that as likely, but stranger things have happened.)
Eventually, there will likely be another sensor design breakthrough -- but I doubt that such a revolution will happen very soon, and it won't likely be the weird camel (ie, a horse designed by a committee) that would be the result of trying to transform a current NEX into a multi-purpose full frame hybrid.
