I imagined that the final product of these contests would be an expansion that, as much as possible, resembles a real dominion expansions. As you'd know if you read the secret histories, these expansions have size constraints.
Let's pretend that the final result of this contest is a 300 card "big box" expansion. If these next 4 challenges have a single winner, and none of them are victory cards, then we have 20*11+3*13=259 cards in a theoretical box for this set. This leaves 41 cards
I always found the constraints Donald X had to work with interesting, and if you look at the secret histories, they actually played a large part in what cards were and weren't included in the game.
It probably won't be possible to force a round number of cards in the final expansion, even if we go up to a 500 card box (which, IMO, we should).
Possible ideas for how to fill out the remaining cards:
*Another card for gaining prizes as one of the challenges, combined with new prize cards as a supplementary challenge
*A supply pile with deliberately fewer cards (similar to how rats has more)
*A "supply pile" consisting of a single card, where buying the card produces some kind of effect instead of gaining
*Replacements for any number of starting coppers (similar to shelters)
*A new "base" card that's included in games with cards from this expansion in the supply
*New cards for the top of neglected supply piles in the base game (eg a copper that rewards you if you buy it, a worse curse that gets passed around from player to player)