good question, going through these ideas I keep in mind: "The winds of change originate in the unconscious minds of domain experts. If you're sufficiently expert in a field, any weird idea or apparently irrelevant question that occurs to you is ipso facto worth exploring." [0]
1/ So the idea I haven't done is build a hacker search engine. I'm not sure what this would look like, but I'd bet being able to search through source code at high/low levels might be interesting.
2/ Another idea is I noted on the the Amazon/NES classic thread, arbitrage by Amazon/users resulting in instant sellout. Could this idea be used somehow?