1080p enough said.... For everything you do on your 2 except perhaps some illegal ways to achieve "cable" television you can do it on the 3. The biggest issue really comes down to whether you want to convert your library to MP4.
I'd expect to see 4K movies in the Apple Store before we see a new 4K device. As far as Apple is concerned right now the whole concept of Blu-Ray doesn't exist in the Apple eccosystem and so neither does 4K.
Until we see a policy change from Apple, and a change away from the "Apple doesn't do Blu-Ray" attitude we won't see 4K from Apple.
All of which are available in my TV as well as the YouView STB, so I don't really find the lack of a third alternative for which device to watch them on that limiting. ATV is still great if you use a lot of iTunes-friendly media.
You will expect it when you see support for it for other devices, the Apple TV is a media player for the iPad, iPhone, Mac, etc... When you hear about an update for higher resolution media you will hear about a new Apple TV and not until.
All of this requires a significant shift from Apple's "we don't do Blu-Ray" policy, to one where 4K is accepted as the norm.
At the moment 4K is incredibly new and there are still plenty of consumers out there with large 2K televisions that are perfectly happy not to shell out another $10000 to replicate a device they already have sitting in their lounge rooms.
There won't be a NEED for a new Apple TV until 4k becomes prevalent, if you need all the other stuff just install Plex Connect.