Apple: Apple failed miserably at making consoles in the past, but that could change if they integrate it into iOS, and their other services. They make good (abeit expensive) hardware, and with the 3rd party suport that they already get on iOS, they could make a decent console.
Google: With YouTube, Android, Google+, and Google TV, Google is slowly taking over electronics. They aren't a hardware company, but they could always outsource the hardwarre to Samsung. I think they would be a major contender in the console biz. They are hiring for an organization called "Games at Google" so maybe they are planning to enter. Google could even do a better job then Apple.
Sega: Sega really cleaned up their reputation with the DC, and they still have a strig following. They could retur to the console biz, but they will need to show the customers that they care, or it will be Dreamcast all over again.
