Circuses can provide an interesting setting for many game genres, from fantasy through to science fiction. Troupes of travelling performers who set up shop for a brief time in a town and then move on are a constant across cultures and times. Because they travel, they provide a good cover for people who need to travel for other reasons - either to carry out some mission, or to remain hidden from whoever is trying to catch them. So circuses can be both a refuge for heroes and also a cover for villains.
The circus coming to town can be a special event in a long-running campaign, triggering various different types of adventures involving the performers. Or an entire campaign can be centred around a circus, with the heroes travelling to different regions to see the world and getting into adventures that way.
For more inspiration on using circuses for one-off adventures in games, refer to Circus Episodes of various media, and for both adventures and campaigns check out Circus of Fear (particularly the Tabletop Games examples).
Transcript
Fozzie: Hey, Mark, you need to be practising your song and dance.
Luke: Why are you looking at me?
Fozzie: Because I'm talking to you.
Luke: First, I'm not your mark, and second, what kind of circus is this place?
Fozzie: This is no circus. This is deadly serious. We have a job to do and you know it as well as I do.
Luke: Uh, okaaay.
Fozzie: Now step up! Loosen your feet! Take centre stage! Bring it on! And dance your little heart out!
Luke: Uh... these feet were made for walking.
GM: Make a Dodge roll.
{Luke Dodges and exits the backstage area through a door marked "EXIT"}