I have an A8/5200 emulator for my Dreamcast. It appears fairly straightforward to me that a peiceof hardware could support all the variations too.
I sense that this is what would be the case if the FB3 were to be A8 or 5200 based. As for dual cart slots, I think that a flash card socket would be more likely. Atari could release roms in this form factor, yet the console could also read a ram flash card if the right Data were on it...such as homebrews and hacks copied on them from a PC.
I think this would serve the public best. It would allow all of us that have old disks and such to use them again on a more modern peice of hardware.
I would hope the thing has SIO capability and maybe IBM PC keyboard compatible too.
If FB3 were 7800 based, I simply don't see the market. The FB1 already did this, and succeeded with regards to the general public who doesn't currently have a 7800 like most of us do (where the sound differences didn't matter). Ditto for the FB2. The FB2 is nice, and will be great for the same market, plus it serves us atari agers too with the extra games and the hackability.
I think a 5200/A8 machine is the most promising. If it has the ability to play all the related formats and use currently existing flash card technology, then the market for it is huge, even as a stand alone regular production machine to sell alongside the PSX , Xbox, etc. Add developers, and the sky is the limit.
my 2 cents.