While it looks like the big Sigma announcement may just be a lens mount service the possibility of something bigger certainly created a buzz. So with that here's what I would have hoped for.
A DP1.5 Merrill with the following features:
A 23.5mm f1.4 lens (or f1.8 or 2.0) lens. (I really love this P.O.V.)
A built-in 2.4 mega-dot OLED EVF.
A larger battery or a grip with a battery option.
A fully-articulating sideways mounted LCD (like what Panasonic G and GH cameras have).
While FF would be nice, I'm actually okay keeping the sensor size at APS-C. And while I'd certainly love to see better noise performance at higher ISO's I'm also actually okay with current issues which limits color shots to ISO 400 and lower. BW I find exceptional to ISO 3200 (when you isolate blue channel information using SPP). Having a faster lens would certainly help. (In fact, using it for BW you'd have one inexpensive challenger to the Leica M Monochrom!)
Just some thoughts. (It's probably way to modest and could go much further. But I'm a simple sort.)