I think it would be pretty dumb for Sega to jump back into the console market. Of course I'd love to see it happen just to see a Sega console again, but I can't see them really making a dent between Sony, Nintendo, and Microsoft.
That said...maybe it doesn't need to be a big console, what if they decided to go the handheld route again by bringing back the Sega Game Gear? A cheaper price point, established old school branding, they have a big library of classic and recent games, that could be something. Though they would likely run into a lot of the same problems Nintendo does in trying to get big modern games to run super well on it and I don't know if a new handheld would work out since Nintendo has basically been the only company to be crazy successful at it other than Sony was the one time with the PSP. They could also push for mobile games to be on the system and if they offer a lot of good features, especially stuff that Nintendo has completely failed at, it could be enticing.
I think the idea of re-branding Microsoft consoles over there with Sega wouldn't be a terrible one, it certainly couldn't hurt Microsoft to do with how bad Xbox does over there and it probably wouldn't be a thing where Sega loses anything from it if Microsoft is licensing the companies name for it.
I'm leaning towards some big new game initiative, like they are buying or creating studios to make a bunch more games, essentially what Microsoft did, just without it being system exclusive stuff. I'm not quite on board with the idea of them going system exclusive, I feel like they make more in general being on all 3 systems and PC and not just 1 system and PC, but there is nothing saying they won't or at the very least go timed exclusive.