Alas that is not a niche though. That is just a design / business choice. By niche I mean a search engine that searches in, say, weather data, just dvds, just computer components, just cars etc etc. Most the time they turn into comparison sites, but to have search engines that just search for data about a particular subject would be interesting. Rollyo, the google verticles, google custom search all kind of does this, but you have to hand pick the sites they are using which kind of defeats the point. Some good symantic programming, verticle analysis and word association algos could product some interesting search engines that don't try to index EVERYTHING and also provide a better chance at succeeding.
Just a thought