Themes should be short, at most a sentence, and should have a broad range of interpretations, and that are inspiring rather than restrictive. Things that aren't allowed: narrow themes that pretty much spell out the entire game (i.e. "Getting drunk on a spaceship with feline aliens"), themes that are basically fetishes ("BBWs", "incest"), themes based on existing fandoms ("At the X-Men Institute"), boring themes ("blondes", "girls", "fantasy", "anime", "college"), or themes that break contest rules ("parser", "pictures only", "extremely long games").

Some examples of good themes:
-- Premise: "Snowed in", "spin the bottle", "time warp", "best club on campus", "mistaken identity".
-- Challenge: "Switch perspectives", "massive twist", "combat"
-- Character: "Nerdy girl", "vampire", "Valkyrie warrior"
-- Setting: "Deserted island", "haunted house", "locker room".
-- Line (something off the wall, open to interpretation): "Sorry, but your princess is in another castle", "But that's not supposed to go in there!"

What counts as a good or bad theme depends on the interpretation of the contest runner.

A NOTE ABOUT GENDER
A theme that indicates gender (i.e. "Your princess is in another castle") can be interpreted to mean the opposite gender ("Your *prince* is in another castle"), if that's what you prefer.