Here's a rule that hasn't been broken yet: All variable-worth victory cards depend only on the contents of your deck; and they never decrease in value from adding more cards to your deck.
Examples of ways a victory card might violate this: (Here, X and Y are numbers.)
If there are 3 or more empty supply piles, worth X victory points. Otherwise, worth Y victory points. [Or you could do finer distinctions based on how many supply piles are empty.]
Worth 1 victory point for every X cards in the trash.
Worth 1 victory point for every X cards in the deck of the player with the [largest/smallest] deck.
Worth 1 victory point for every X kingdom cards left in the supply.
Worth X victory points, minus 1 for every Y [type of card] in your deck, to a minimum of zero.
Worth 1 victory points for every Y differently-named cards in anyone's deck.
...and so on; I've left out lots of obvious variants on these.
Some of these are probably terrible ideas, but I suspect not all of them are, and so far none have been done.
How about something like:
Worth 1 VP point for any positive number of cards in your deck distracted from 15. (May be expressed weird, I am not a native speaker.)
The idea is that the card gives you 0VP if you have 15 (or more) cards in your deck, 1 VP if you have 14 cards in your deck, 2 VP if you have 13 cards in your deck, 7 VP if you have only 7 cards in your deck and 14 VP if it is the only card in your deck. The number (15) may need some tweaking. 20 would probably be to much, 15 not enough.
Another idea I recently had is a card with a cost of $6 and the addition: "If this is in the supply every player chooses his starting hands and may play them with any number of cards (but never more than 10 cards in total)"
So you can basically start with hands of 7/0, 6/1, 5/2 or 4/3. But as you can choose the amount of cards up to ten you can also start with things 4/0, 4/2, 3/0, 3/2, 2/0 (or 0/0
) and draw the spare coppers or estates you didn't use guaranteed in the third turn.
The wording is probably poor, but I guess you get the idea of being able to use any number of cards from your ten starting cards in you first two turns. If you choose to start with all 10 cards you do not shuffle after your first turn; you play your second starting hand with 0 cards. If you choose to start with 7 coppers you play your second with three (or two, or one, or none) estates and no other cards. etc.