Well, an interesting one might be a knowledge game including live web searches and results.
The engine could load some webpages and generate/update questions by itself (e.g. from news), which the user has to answer correctly, or at least narrow down the search queries.
Something fun and educational, where you might even improve your $searchengine skills.
From a programmers view it might involve intelligent parsing capabilities to generate Q&A and check correctness of given results.
Whatever, KISS might be a good start ;)