Similar to the above -- do you ever look at fan cards? If so, do you have a favourite?
I don't usually look at them, because 1) I don't want people feeling like I'm taking their ideas, which probably I had years ago, not because I am amazing but because the obvious ideas are obvious and I had a big head start; and 2) the cards that aren't in sets already are usually awful, nonstop things I wouldn't do that are boring and redundant or else obviously bad for the game in some way, and if it's not obvious then I already tried them and found out the hard way. At best they are things I'm already doing; none of it is good reading.
Let's do an experiment, I will look at the first four cards of "Books of Magic," the first listed fan expansion at BGG. I am looking at the first four images sorted the default way ("hot"), skipping the big image of a card sheet.
Ghost Town: My first version of this was called Fool's Gold and was "+$2. Put this card into your hand." That turned into a card that gave you +$2 and an extra +$2 for each unused action you had when you played it, then +$1, then I made it into Diadem.
Book of the Dead: Getting something from the trash and saving a card for next turn were the two most suggested card premises ever (prior to those cards being published), followed maybe by a reaction to punish attackers. This card manages to put the card on your deck like Graverobber does, but of course is missing the crucial "provide a way to get stuff into the trash that you'll want" part.
Fairy Gold: The first version of Feast was this only with +1 Action, for $4. It was too strong and turned into the Feast you know. I eventually did a one-shot Gold that you can't buy, with Spoils.
Gravedigger: I haven't actually done giving yourself a Curse in a published card, although Death Cart gets close. I tried multiple cards that gave yourself a Curse; everyone hated them. Death Cart dodges the problem by giving you a use for the Ruins.
So, four-for-four, nothing new or interesting here. Sorry Books of Magic guy, it was just an example.
Sir Bailey made Courtyard, but he managed that because he showed me his homemade cards when I didn't have that many homemade cards of my own yet. Even so I had already done "+2 cards +1 action, put 2 cards from your hand on your deck," but abandoned it because it played so slowly. Dame Josephine similarly managed to get Counting House in relatively early on.
I will cite two favorite fan cards though.
As I said most are awful. The stand-out awful card, the epitome of awful fan cards, was one called Locusts that read "each other player discards a Gardens." First the guy of course must have meant "trashes a Gardens card from his hand" but blew it and ended up with the most useless thing ever. If he had gotten it right then it would still be crazy awful. He started with the flavor of "locusts destroy plants" and did not think of "wait you have to want to play the card though." The old Magic expansions Legends has stuff like this, where they made flavor-based stuff and just did not consider, why would anyone play this.
There has been to date exactly one card I saw where I thought, hey, cool idea. It was something like, "when you gain this put it in the discard pile of the player to your left; at end of game worth 2 VP for the player to your right." I have done hot-potato cards that did not work out and probably this would fail for the same reasons, but still, neat idea.