I love the "wild card" space, which is how this is usually treated in creative bingo. You can put anything in that square, or according to some fests, pick any prompt from their list. It's not typically a square that doesn't have to be filled, the way it's sometimes played in gambling or educational bingo games.

On that note, a three-way option among "free space," "wild card," or "normal prompt" as the center square would be helpful.