Not a single one of them.
Entering the hardware market is simply too cost prohibitive now. MS got the last seat at the table.
To be successful would require a sizable internal studio and lots of 3rd party and retail support.
1. Getting a sizable internal studio would be very tough short of buying out a bunch of studios and those studios are coming with huge price tags.
2. 3rd parties are already spread incredibly thin providing software for more platforms at once than ever before (2 Nintendo, 3 Sony, 1 MS, PC and Mobile = 8). They will be highly reluctant to spread themselves even further to support an unknown entry.
3. Retailers have limited shelf space which to support those 10 aforementioned platforms. Try asking them to make room for a 4th major home console and they'll laugh you out on your ass.
The rEVOLution is not being televised
