This is happening with Android already (Ouya) but Apple doing it would be a bigger paradigm shift in the industry as 3rd party game developers of all strips and sizes go ape sh-- to cater to their 300M strong audience.
Many will argue 'The core games aren't there' - If Apple opens it's door to core gamers in this way, the core games will most assuredly be there. And so what if it doesn't have exclusives? (it will btw) 360 barely has any anymore, MS doesn't seem too worried.
But specifically:
WiiU will suddenly look like a cheap (and overpriced) Apple knock-off and be in deep, deep trouble as it fails to grab any casuals (or core)
Xbox: IF all the virtual/kinect rumours are true, MS will be just fine. They'll stick to their current plan.
Sony: Innovate or die. PS may just become a super gloried game service available through other Sony devices.
Edit: I have zero expectation of this happening, however, there is a slight chance it would so Android doesn't get a solid grip on that market segment.







