That's an interesting topic of discussion at NeoGAF right now. Obviously, if it were up to me, I'd bring back the Dreamcast. I wouldn't even bother with modern technology or new designs - I'd just bring back the good old Dreamcast we knew and loved a decade ago. Of course, I would probably sabotage everything in a Don Quixote bid to bring back the Saturn, this time with a ton of RAM and an emphasis on 2D video games. By the end of the month, I'd probably be standing on street corners selling pencils.
Obviously, hardware doesn't matter when we're talking about Sega. People loved the Genesis, Saturn and Dreamcast because of the great arcade games. If they'd just bring back those old school video games, everything would be fine. But I don't see a lot of action on that front, aside from the download services on the three major consoles...oh, and this little thing called Apple. And there's no denying that Sega lost most of its mojo after getting out of the hardware business.
Come back, Sega! All is forgiven! We're not angry about the Sega CD and 32X anymore. Bring back the Dreamcast, bring back the NFL2K series, and bring back Capcom's CPS2/3 arcade fighters.