Would DDG consider being a partner with Archive.org to automatically include the results of its databases in your searches? In particular, having the ability get the text results from its book scanning project of public documents would be very ducky. Having access to texts is one of the things that I liked about Google (Google Book Search) and that I have missed since going over to DDG.
But the Internet Archive offers much more than books, but other digital information. Also DDG could use the Archive's WayBackMachine with date settings to search how websites used to look.
Lastly, for this round of suggestions, perhaps DDG can incorporate Patent information from U.S. and foreign patent offices directly into searches, perhaps with a Bang option to just search patents only if the user did not want any non-patent results.