That's a great idea and consistent with rumored AirPort Express supply constraints. It'd also be a nice way to start to eliminate Apple's Broadcom/MIPS wireless device codebase and standardize on iOS which in turn is a Mac OS X variant, so it'd make a fair amount of development sense, too. It'd also be configurable without requiring the AirPort Utility -- just hook it to a TV and use the remote -- and that simplicity is consistent with Apple's philosophy. And it'd be a gentle nudge to encourage people to hook up their Apple TV via Ethernet if possible. Also cuts down on SKUs and helps reduce Apple TV problems associated with dodgy wireless routers. And it's a nice competitive differentiator.
Maybe they'd call it the "Apple TV Express"? Basically it'd be an AirPort Express with upgraded wireless (802.11ac), an added HDMI port, and Apple TV internals (probably upgraded to single core A6). The existing USB port could also be used to access audio, video, and picture files from a USB memory key or USB hard disk.
I like it.