Apple developing an entire TV makes little sense to me. TVs are purchases that people expect to last 10+ years. I've had my current TV for 6 years and have no plans to replace it anytime soon. Apple's business plan is for users to upgrade their hardware every 2-4 years. People are not going to do that with a TV.
I think they have the right idea with the current Apple TV 2. A box that uses your TV (any HDMI TV you choose) as a monitor. If they would build that idea out a bit (thinking a few more apps) and maybe some streaming deals and allowing Siri like control via their Remote app, then you have everything that a full Apple TV could do in a little, relatively inexpensive box that can be upgraded every 2 years with out much hesitation from the user.
If Apple did build a TV, it would be gorgeous, nicely integrated into the Apple ecosystem, but it would likely cost upwards of $2000 or more and only come in one or two sizes, which may be a turnoff to the TV buying public.