DuckDuckGo isn't a search engine for Tor, it just provides a hidden service interface to its regular search engine. If you want to make a hidden service to help improve the overall quality, let me know and I can help you get it all setup.
I thought you could limit searches to .onion domains. Oh, well. Thanks for the offer to help. There are a few ideas for site that I would want to see on the Tor network, but I don't think they would have the draw enough traffic to get people running TOR on the regular.
Though writing this gave me an idea: is there a .onion Torrent index?
You can download the entire PirateBay database of magnet files, in under 100 MB. If you want to provide a robust place to grab torrents, a hidden service is a good idea.
Beyond that, I think that dedicated TOR users will shy away from using BitTorrent since it's a huge privacy risk. They're more likely to be using FreeNet or I2P for anonymous P2P filesharing.
