2 key factors to consider in all these speculations is 1)Lens to flange distance 2)Mirror size.
Both are of course related to each other. 2) is obsolete IF Pentax goes with a total EVF (electronic viewfinder) which I consider unlikely.
Next major thing to consider is it is anti-pentax to make things bigger than they should.
Sooo this leave only a few full frame possibilities from these "assumptions"
1) FF sensor in a typical SLR sized body. Adapters for 645 lenses as usual.
2)FF sensor in a 645 size body (unlikely due to above considerations). Only 645 lenses need apply
3)FF sensor in a modular back 645 body w/ full frame and mf sensors available.
4) MF only in a smaller body (645 lenses need adapter, ff/aps lenses still don't fit)
The aps-c and full frame lenses have the same lens to flange distance. VERY unlikely.
645 ect. are much longer so they fit on a dslr w/ adapter.
Unless Hoya changed their mind, a ff is directly fighting w/ C/N/S, which from their prev statements they ar trying to avoid.
My bet still rides on a 645 w/ modular back. Full frame sensor standard equip. (I'm a terrible gambler)