I have an idea that (hopefully) shouldn’t be difficult to implement. How about site specific browsers? To get an idea of how they work, check out the Mac app, Fluid. Another possible implementation could be “web clips” ala iOS:
http://www.apple.com/webapps/whatarewebapps.html
There are lots of web applications that would work beautifully in a site specific browser.
Online sites that should work well:
EyeTV’s streaming web interface
Pandora
facebook
mint.com
Vimeo
Home Automation Appliances / Server Apps that run in a browser:
Mi Casa Verde Vera (home automation device w/ web interface)
Irrigation Caddy (sprinkler system w/ web interface)
SABnzbd (Sick Beard, CouchPotato, Headphones)
uTorrent web interface
Sites specially formatted for TV:
Mobile sites (some are prettier than others, but usability is high):
m.foodnetwork.com
Bank of America’s mobile site (bofa.mobi)
a multitude of other mobile sites available @ http://www.apple.com/webapps/
I’m sure there are plenty more that I’m not even thinking of right now. It’s home automation and server apps that get me most excited, though.
Site Specific browsers would basically allow users to create their own apps from any website they choose. That would be the ultimate in extensibility. And aTV Flash already includes a web browser so I can’t imagine it would be so hard. 
(… at least I hope not.)