Once again, gaming on the Apple TV is something I'd absolutely love to see; Apple already has the app store infrastructure, all that would be required is a hardware update for the Apple TV (which it's really due already) though support for the current generation wouldn't hurt if apps can be loaded via a computer (to overcome the low capacity). A bigger capacity drive so the device can function on its own is the main hardware requirement, though of course a better GPU and a CPU bump wouldn't go amiss.
Things are pretty simple on the software side as well, as the main thing Apple needs to do is provide an API for a controller, and they could easily release a controller app that allows you to use any iOS device as an ad-hoc controller in a leu of a proper physical one (which could be left to third parties to produce). Game Centre covers things like multiplayer and achievements, so there really isn't much else that's needed.
It really is a fantastically small amount of work since the hardware is something that would presumably be upgraded anyway, and the software is mostly just an API away from being gaming ready, though there are other things I'd love to see (e.g - support for running a big version of an app on your Apple TV while running the same app as an interface on your iOS mobile device, with the ability to pick up save games and take them with you). Opening up the Apple TV would also have the side-benefit of eliminating the need for Apple to keep apps for Netflix, YouTube etc. up to date and the services can do that themselves (though I believe some may do-so already, which suggests some kind of dev kit already exists?).
The Apple TV would slot easily into the same kind of budget indie gaming space as the Ouya, except that Apple could potentially provide it with superior hardware to give it an edge. Porting apps also wouldn't be that hard as anything supporting the high-res iPad is already rendering at better than 1080p, so it's really just an issue of adapting controls to support controller style commands.
Controller awareness is something I'm dying to see anyway, as I'd love to have an adapter that can turn an iPhone/iPod Touch into a Vita style handheld, and if you could just sit down in front of an Apple TV to switch to playing on a big screen then it'd be great.
I dunno, analysts keep saying this, and Apple keeps staying silent, but it's an opportunity to add huge value to the Apple TV with a tiny amount of effort; I mean, you could hash out a controller API in about a day at which point you're most of the way there. Add Apple TV to the iTunes store, release a new model with the latest hardware and write up a simple controller app for portable iOS devices and you're done.