I think the first step is to harvest domains. Start with the domains that are by far most popular as a seed for the database. Then I can see two probably not

I think the first step is to harvest domains. Start with the domains that are by far most popular as a seed for the database. Then I can see two probably not very good idea. If the user does not get a hit on their query for domain, the search engine would query bing/google/or other and present a list of possible values. Then the engine would learn. It could also be a browser plug in that records domains and search terms used. There is no reason to record who sends it just what the values were. Given a lot of people running it across different countries it would slowly build an index. Since this is a privacy violation it would probably not be popular. I wonder how long of a list of most popular domains I can find.