In a pinch when out of tartar sauce I’ve mixed green relish with mayo and it’s, passable.
For that matter, mayo with hot sauce and ketchup was my accompaniment to some otherwise bland pizza and tater tots this afternoon.
Microwave Hollandaise sauce. Can go wrong easily, but when I wanted it quick it works fairly well. Just have to tend the microwave every 15-20 seconds to mix it before it coagulates. But it’s done its job more than once for me.
Sour cream works well with basically anything tomato, including ketchup. Or a hot sauce of right consistency, not so hot that you only use a few drops.
For best results, use the sauces cold, and don’t mix them too much — so you taste them both separately instead of blended mush.