One day I was thinking about what good unexplored ideas there are for non-attack player interaction. Then the concept of counting if a pile had an odd or even number of cards in it came to my mind. But I realised that would be too cumbersome in play, so instead I decided it would be better to use a token to keep track of this, by flipping the token whenever a card from the pile is gained, which works the same way barring a few edge cases. Then I realised I could extend the token flipping concept to other triggers.
All these cards have the card type "Seasonal". It means, this pile starts with a Season token on it (face up), and you flip it whenever you gain that card (unless stated otherwise). Also, when a card refers to " the Season token", it refers to the one on its pile (unless stated otherwise).
Card A $2
Action-Seasonal
+2 Cards
+1 Action
If the Season token is face up, discard 2 cards.
Simplest idea. Can be a cheap Lab, but at worse is worse than Warehouse. This card is in the bad state when the pile is empty, so it will not be broken.
Card B $4
Action-Seasonal
+2 Cards
If the season token is face up, trash any number of cards from your hand and gain an equal number of Coppers to your hand. Otherwise, you may discard 4 identical cards for +3VP.
I like the idea of trashing cards and gaining a Copper to hand in return. The top part is a Vault-ish effect, and the bottom rewards an engine, but being in the odd state cannot generate VP forever.
Seasonal Village $5*
Action-Seasonal
+1 Card
+2 Actions
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This costs $3 less if the Season token is face down, but not less than $0. When you gain or trash this, flip the pile token.
Varying costs with the Season token sounds like a good idea, but I can't find a good card to tack it on to. Good with TFB, bad otherwise, even though you still probably want them. (Making this a TFB itself sounds like a bad idea)