It would be neat if Sega re-joined the console market.
I think Sega's time is passed. But if they did I think if they merged with Microsoft's R&D, but I doubt Microsoft would abandon it's "Xbox" hardware branding, even if the hardware was half or all Sega's in exchange for the funding. As an independent company with no other divisions to back it up, I don't see a 4th company doing well. Nintendo does it somehow, but it was one of the first of the current companies. Sony and Microsoft have the financial backing of their respective companies. I'd like to think Sega has enough fans to make it pull through, but that was pretty much disproven in the Dreamcast era and the availability of the Sega software on all current system removes any sort of exclusivity appeal.
In summary: All a Sega console would be now is an Xbox with a different logo.