Is there anything out there that says this 'full frame' camera is 24x36mm? For the purpose of a teaser - essentially, an announcement-of-an-announcement - they could justifiably call anything 24x36mm and larger full-frame. And, if it were larger, the shock value of the announcement would be extreme.
For a 24x36mm sensor, the lens mount is odd. Some argue about the ability to make faster lenses, but Leica have been making f/0.95 and f/1 lenses for decades, with a mount that's both narrower and deeper than the E-mount. And Nikon themselves have had no problem making f/1.4 primes for their deep and narrow F-mount. With a telecentric design, it doesn't matter what the throat size is, as long as it is at least as large as the sensor - all the light is coming perpendicular to the sensor anyway, so any light passing through the throat not directly in front of the sensor isn't going to hit it. In any case, superfast lenses would only make up a small niche in any manufacturer's customer base - for the bread-and-butter f/2.8 zooms and f/1.4-f/2 primes, all the larger mount does is make the lenses bigger and heavier.
But a larger-than-24x36mm sensor could be Nikon's best trump card. Nikon came late to the full-frame DSLR party, and has suffered badly for it - Canon grabbed the lions share of the market by being first, and has kept it. Now they're also late to the mirrorless party - Sony has incumbency here, and starts with a huge lead in compatible lenses (adapted lenses are never really the same as native, except where there is no AF anyway). But going for a larger format - 30x45mm, 33x44mm, even 36x36mm - and defining that as their new 'full frame' would turn the tables on Sony. Nikon could then easily claim supremacy in imagy quality. Many current Sony users would have a major reason to switch systems - after all, image quality is the reason they switched from Canon in the first place. So would Canon shooters, who would have to switch systems to move to mirrorless anyway. And, for a lens manufacturer working with a closed mount system, it would make it much harder for Sigma et al to compete for the first few years, because they couldn't simply slap a Nikon mount on their existing lenses and call it a day. Yes, they'd be competing with Fuji, but, comparatively, that is small fry. Sony would struggle to respond - the E-mount can handle 24x36mm, but nothing larger. Even with an edge in sensor technology, a larger sensor is difficult to overcome.
If Nikon came out with a larger sensor, implementing a crop mode to shoot with current lenses via an adapter and designing new lenses around the larger sensor (e.g. 20-40mm, 32-90mm and 90-260mm f/2.8 or f/3.2 zooms), they could well turn the tables on Sony. Even if the initial release were 24x36mm, but they announced that future models would have larger sensors and designed their lenses around them, it would have a similar effect - after all, early DSLRs were all crop models too. And it would justify the mount size and associated lens size. Otherwise, Nikon could again find itself starting from behind, against a larger and better-resourced rival.
