What would you want to see in a DD app.
So here's a list of randomness...
A built in CC list that doesn't redirect to cardratings or anywhere else until I'm ready to apply, with links to DD latest articles/reviews next to each card.
A searchable section for "other deals" which weren't DDMS worthy, but were posted to DDF.
This is a super long shot, and probably more applicable to DDMS than an app, but a DD user review section for items. There have been a bunch of items I bought or didn't buy because of DDF reviews, where the outcome may have been different by just relying on Amazon or other sites' reviews.
FAQs. The same questions get asked a million times on DDMS. (General DDMS idea - ask a user feature. There are tons of people who will never actually join DDF who ask questions, and plenty of helpful and knowledgeable DDF regulars who are happy to answer. Can be a feeder to build FAQs.)
Built in page with DDF notifications, even if it redirects to a browser to see/read details.