I've been out of the fan cards scene for a long time, so I'm guessing something like this has been mentioned before. My idea is to have non-cards like Events/Landmarks that, when used, change the rules of the game. But, unlike Events, they aren't things you can buy, and unlike Landmarks, they don't just affect scoring. Can anyone point me to where this type of thing has come up in the past, assuming it has?
Here are some ideas I had for such cards. They would all change how the game was played, some more so than others, in a way that might feel a lot like "house rules" or a total game variant. You certainly wouldn't want to play with them all the time, but I could imagine some of these making for some interesting Dominion.
During Cleanup, draw 6 cards instead of 5.
Add an extra Kingdom card pile on top of this card. When you buy a card from this pile, +1 Buy.
Play with your cards visible to your opponents.
Copper, Silver, and Gold give +$1 more when played.
After looking at the kingdom, you may choose the number of Coppers in your starting deck, anywhere from 0 to 10.
(2-player only) When you buy Victory cards, they go into your opponent's discard pile. At the end of the game, whoever has the fewest victory points wins.