I thought about the API - this is a good idea. I was thinking that I also can do a widget that people could put on their website. For example, they could put: "Top news mentioning USA in Finance category". I think that would be really useful for a lot of websites. However, no time for this now. Improving design based on feedback and looking for investors.
I didn't really get your idea about search engine. You can already type anything you want in the search bar at the top. The results then can be further categorized.
P.S. Too bad HN doesn't send out notification when someone replies, I missed your comment three days ago.