There are all kinds of things that you can use mallard flank for, however, there is mallard flank, and mallard flank. The little ones that are often dyed brown and passed off as bronze mallard are next to useless.

Probably the most common use is as fan wings. You can use them in any pattern with fan wings. Bunched fibres make good parachute post. They can also be used for wonder wings. I would use them mostly for Wally wings. Spey hackles, as Allan says, is another use. To really work as spey hackles the feathers need to be "burned" in bleach, so they do not clump together. (Not a difficult process look up "burnt spey hackles" on You Tube).

You could tie them in and wind them for a body. Tie in the stem with a couple of loose turns, and draw the feather through so the tips are captive. Then wind it. Best to rib these bodies as the fibres are not very tough.

There are many uses, the only limit is on your creativity.

I know I haven't answered your question. That is deliberate. A list of patterns that use a material is not the way to go. Many of the patterns came about because people used and adapted what they had. Patterns are not set in stone. Certainly not for fishing. I learned my tying in the north of England with hundreds of years of tradition. I now live in the north of Scotland. Many of the patterns I learned years ago work here, however they work here better if dressed heavier than I would for use in England. Patterns should be changed and adapted to fit your circumstances.

Here is an example of the flies that I was using years ago (top row) and how I combined them into what has become one of my favourite patterns. As I discovered other flies and tying techniques (left and right) they influenced the fly in the centre until it became what it is today. It is not finished. It may become something else tomorrow.
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This is why I've given you techniques rather than patterns. I hope it helps though.

Cheers,
A.