SNK - they made good consoles, they just ran into the price snag. If SNK made another foray into a home arcade system it could go quite well.
Konami would be another good company to get into it, if we're talking a company that hasn't been in the console business before.
It wouldn't matter terribly who made it though, since the GPU would be either nVidia or ATI and the CPU would be IBM. Then it's just a matter of putting it together in a way that wouldn't follow the 360's hardware failure, and then hand over control pad manufacture to a company that does it well.
Really though, is there room for 4? Sega bowed out when Microsoft stepped in, what company could muscle their way in?