Anything you’d consider lard for, aside from pies, and just about everything you’d use butter for. I rendered four large Tupperware bowls of goose fat (de Bresse) and have used it for everything from stuffing to cassoulet to eggs to tossed pasta with chicken. Crispy potatoes with eggs is just a gimme 
Got an Instant Pot? Duck fat risotto:
+1
Watch “Last Tango in Paris”.
Oh, wait, that recipe used butter.
Matzo Brei. My mother made it with chicken fat. Duck fat has to be even better.
Cook shrooms and then drop em in a risotto!
Might also consider duck fat infused bourbon or rye. I’ve done it with bacon fat several times and used it to make Manhattans and Old Fashioneds. Works well with anything with simple sugar in it. Might be fun with a mint julep.
Mushrooms and duck fat are indeed a great combo.
A small bit is great for finishing seafood dishes, especially scallops.
I always swirl a small spoonful into cream sauce for shrimp and grits.
I like to cook bacon in duck fat.