A masquerade mask works as a standalone accessory, but it also pairs naturally with a handful of costume themes that were basically built around wearing one. If you're trying to build a full look rather than just add a mask to your normal outfit, these are the easiest starting points.
Phantom of the Opera
The most direct pairing: a Phantom-style half mask, a black cape or long coat, and formalwear underneath. This works especially well for men and doesn't require much beyond what most people already own for a formal event.
Great Gatsby / 1920s
A flapper dress or dapper suit with a feather or rhinestone-accented mask in gold or black leans into the Art Deco glamour of the era without needing a literal period costume.
Venetian noble
For the most historically direct option, pair a full-face Bauta-style mask with a dark, floor-length cape and, if you want to go further, a tricorn hat. This is the most dramatic and most literal "masquerade" costume, closest to the original Venetian Carnival look.
Gothic or dark romantic
Deep jewel tones, lace, and a more dramatic silhouette. A black or deep red lace mask paired with dark, structured clothing reads as gothic-romantic rather than costume-y, and works for Halloween as easily as a themed masquerade ball.
Keep it simple
You don't need a full costume to make a mask work. For most masquerade events, formalwear you already own plus the right mask is enough — the costume ideas above are for when you specifically want to build a complete look around a theme, not a requirement for wearing a mask at all.