The question is simple, what most users would like to see as themes for the contest.
1) A specific theme would translate in "Serial Killer" or "Horror" and wouldn't have any required elements.
2) An open theme would be wide open, without anything specific to be related to but would need to include some required elements, such as "A burning candle" or "A glass of water" somewhere in the short.
3) The last option would be, at the request of the winner who choose the theme, a mix of both previous option: Specific Theme With Required Elements. That would make it more tricky and challenging.
What do you like most?