A strange thought just crossed my mind. What if Apple, rather than combining the AppleTV with the the Mac mini, were to instead combine it with the Time Capsule? The result would be an AirPort base station, plus a largish, reliable hard drive, plus video hardware and ports to allow wired connection to your TV or home theater setup. One product, three complementary uses. Make that four complementary uses, as such a device would surely also run iOS Apps, including games.
The hard drive could automatically be divided between maintaining a media library and preserving Time Machine backups. The AirPort functionality would work equally well to stream media to another device, or to allow you to connect controller hardware for games, or enabling your laptop or desktop to send incremental TM backups, or simply as a means of connecting to the internet.
Really, if your starting point is the AppleTV, all that's needed is a faster CPU and GPU, something adequate for games, a larger hard drive, and the software in a Time Capsule. The A4 chip is probably an adequate CPU, although a dual-core variant or something based on the next generation ARM design might be preferable given the need to drive a 1920 X 1080 display, but these parts shouldn't be prohibitively expensive.