I agree completely and fully with everything Marko said.
However, if I were going to recommend you improve the web interface of MC, I would urge you not to waste any time trying to make the included internal IE any better. Its just silly. I agree with him. I have a browser (two actually) installed. If I want a browser, I'll use a browser. I wish MC would just work with my default browser instead of trying to handle HTML assets internally, honestly.
That's not to say you couldn't do something useful with it. I'd concentrate on making it HTPC ("10-foot") accessible. I think this is where MC has a real "niche" opportunity, if you want to do anything web-focused.
I can use Firefox or Opera on my HTPC, but they still clearly aren't designed for it. Even with my big, widescreen TV... It is still bad, doesn't make good use of the screen space that is available, is hard to read, etc, etc... A more 10-foot focused browser, with zoom functions (ala Opera) well implemented, remote-based page navigation scheme of some sort, big-easy buttons for functions, integration with Theater View, and improvements to the great RSS reader Matt built a while back (I'd still like custom sites if somehow possible). That's where I'd put my web development efforts..
Not into re-inventing the wheel, or making a cheesy start-page that no one uses. Web "portals" are so 1998, and they weren't even that cool then.