The roast duck sandwich is the richest member of the roast set, and what defines it is fat management. Roast duck is a dark, deeply flavoured meat carrying far more rendered fat than beef or chicken, and cold from the bird that fat firms into the slices and onto the palate. Left to itself between bread it reads as heavy and one-note, lush but cloying by the third bite. The whole sandwich is built to keep that richness in check: sliced duck breast laid on bread with a sharp, sweet sauce, plum or hoisin, that cuts straight through the fat with acid and sugar at once. The duck supplies the depth; the sauce is the part that makes the depth bearable across a whole sandwich.
The craft is balancing the fat rather than hiding it. Duck breast is sliced thin and across the grain so each piece carries a controlled amount of meat and fat instead of a thick band that overwhelms, and the sauce is laid in a measured stripe because plum and hoisin are intense and a flood turns the sandwich sticky and the bread to a sweet patch. A crisp, watery counter earns its place against the richness, shredded cucumber or spring onion, the same vegetables the Chinese pancake tradition pairs with duck for exactly this reason, breaking the fat with cold crunch. The bread is plain and reasonably sturdy so it carries a heavy filling without collapsing and does not compete with a meat that is already loud, and butter is light or skipped because the duck brings more than enough fat of its own.
The variations are mostly which sweet-sharp sauce against the same rich meat. Hoisin pushes it savoury and dark; plum sauce takes it brighter and fruitier; a sharp orange or cherry sauce leans on citrus or stone-fruit acid instead. The hoisin-and-cucumber build folded into a flexible wrap or pancake is its own format rather than this bread one, and duck with orange as a plated-leftover sandwich is a closely related reading. Each deserves its own article rather than being crowded in here.