I would dearly love to see Sega back too, but they would have to release the most massively overpowered machine to even get their foot in the door. I seriously doubt that any company wading in to the market by releasing a console of similar spec to the current competition would entice any buyers, you'd have to burst out of the gate and hit the ground running.
I think they should bring back cartridge gaming! Also, I truly believe that if Sega were to make a come back then they should drop Sonic, he ceased being cool in 1994. Just drop him entirely, no rebooting of the franchise, no 2.5D attempts to claw back some of the glory of the originals, just put him in the garden, give him a saucer of cat food and slowly close the patio door forever.