With the basic concept of a TV, pretty much nothing. It is a dumb display that shows content, aside from spec bumps (better backlighting or viewing angles or resolution) there isn't a whole lot to do there.
Something that Apple could do, whether or not it is feasible doesn't matter, it is something that they could do that few others could, would be a complete bottom up rethink and redesign of how we get content and interact with said content on those systems. Imagine an Apple designed cable box, one that is easy to use, and doesn't blow. Integrated into the rest of the ecosystem, you could take that same content and watch it on any iDevice (remember that post-iCloud that includes your iMac/MacBook now). It isn't about building an AppleTV into an Apple TV, it would be about a complete overhaul of the cable industry, not unlike how they changed the music and cellular industries.
More digital sales of music, more buying songs instead of albums, and the slow abolishing of DRM on their music. For the cellular industry, they have helped to take at least some of the power away from the carriers, and made the updating process so much easier. Plus the explosion of data use that (hopefully) will lead to more positive change for the customers.
I don't think that they will do it though. Cable seems to be entrenched in a way that not even cell phones were, with just one provider in a lot of areas, and best case scenario, maybe 2-3 in highly populated areas.