Alternative for cooking duck with plums?

Prunes would work well. They are dried plums. You could rename them to make it sound more elegant and less Grandpa-ish. :)

my grandma makes this slow roasted duck with apples which I vaguely remember as very, very delicious.

I made duck alla porchetta for Thanksgiving and thought it turned out very well. The recipe called for duck breast, but you could adapt it for legs. Mark Bittman has a recipe and there are several others on the web. I took ideas from several recipes for my version.

I also think it might be interesting to do duck with pomegranate--you might google that idea and see where it leads.

My mother used to make a delicious duck with grapes. Roasted grapes are lovely and I'd think they'd work well as a plum sub. Just make sure to use seedless.

Ducks with dates are delicious!

I was going to suggest quince, as lemon ade has. Also, if you have a well-stocked freezer or grocery store, look there...nowadays all kinds of fruit is frozen in peak condition. E.g. mangos or peaches would be great on duck.

Figs and oranges together. Sweet and tart.

Apples are lovely as well!

I think quince quarters would be nice, with a bit water (or wine, sherry, cider, stock) added along with them, since they won’t really release too much juice. Or how about orange slices (peel on, if you find thin-skinned, organic ones)?

Oranges. Duck a l'orange is classic and yummy