Obviously, the easiest is to build on top of existing index. This in turn makes it a purely marketing play (DDG's marketing play is "privacy").
Here is one exploration of the following concept: search engine built on top of high quality sites only (as vetted by HN submission history)
https://cse.google.com/cse?cx=014479775183020491825:c2lrlzro...
Described in full here: https://news.ycombinator.com/item?id=21209358