I was thinking of having a crawler visit all the pages on the dark web. But there's no broad similarity in the way the websites are named.
Actually your suggestion is what I started before. The main problem is the seeds. We do not have all the onion addresses. I recommend you to read my project details named OnionHarvester: OnionHarvester
Please do not attempt this
The entire purpose of hidden services and such is privacy. You suggestions would destroy that. A search engine to find the supposed to be unfindable? A paradox in itself. Some of these services are hidden for a very good reason, such as humanitarian organization under oppressive regime. Your search of the dark web would help the oppressive regime out a lot
If you really need to access a hidden service, contact the server operator. Some services do already offer something similar, like this
Hidden service are hidden and by design of the tor network should remain that way