Wild predictions:
Hardware:
Its a box.
It has 3d sensors, principally for facial recognition/user identification. This would allow for no passwords, multiple users, and parental controls. It may also have gaming applications.
The main input device might be an ergonomically shaped remote trackpad. (An "air mouse" or "iRing" is likely too difficult to use? An ipad or iphone is problematic because your gaze would be shifting between two screens. Who knows, I'm sure they spent millions figuring this out.)
UI:
A full version of IOS.
Content:
I'd love to see them avoid the cable companies. The writing is on the wall and perhaps media network loyalty to cable companies is waning. Content could be provided directly from them, purchased through the app store via monthly subscription. Wont be al'cart channels, unless the network permits it on their own volition. HBO, ESPN, Hulu, Netflix, NBC, CBS etc. Shows become available as they air. Potentially with more costly ad free options.
Live TV:
This will be interesting, but I think the burden for building the infrastructure to support it will be on the networks, not Apple. It will be apart of that networks app.
Gaming:
Purchased via ap store. Controllers are 3rd party.
A full TV set:
Maybe years away, but if we see one, it might be transparent when off. I don't think this would be a gimmick, I think it'd be an example of Apple wanting the "tech to get out of the way."
Home automation:
May act as a central hub for controlling 3rd party low energy Bluetooth devices such as light switches, thermostats, surveillance cameras, locks, etc.
ISP:
Same old cable companies, but it will eventually be fiber.
Within 8 months, Google releases an android TV version, with hardware produced by 3rd parties. Cable companies begin to loose billions in revenue, their stock prices become volatile, companies are bought and sold, some consolidation occurs.
Ultimately we'll pay more for internet service, and more for content, but we'll have greater choice and convenience.