Well, it seems like we are all thinking along similar lines. Certainly, a full frame 'SD16' or similar would be a force to be reckoned with and might enable Sigma to pull out something of a lead in this area.
I would imagine that both landscape and portrait photographers could put such a tool to very good use and, of course, there are some quite nice Sigma lenses ready made for such an instrument - although it might also inspire some interesting R&D in optics as well. There are also, no doubt, various specialist areas of photography (scientific etc.) where a full-frame foveon might redefine the affordable baseline.
I wonder if anyone at Sigma is reading this thread...
Kind regards to all,
JA