It may be fun to try to figure out how to implement these IRL. Some of these implementations may still suck, but Donald said that just being bad IRL isn't enough reason to make it as an online-only card (
here).
When I was trying to come up with the simplest possible such card, I had:
???: Treasure, $?
Worth $1 per time you've played this (counting this one).
Each card in the pile has an identifier (A-J or 1-10). The card text says to increment the corresponding counter when you play it. The counter could be done with tokens, or just tally on paper or something.
A card could access hidden information (like the number of a certain type of card in your deck) and produce a resource based on that. For example:
Collectible
Treasure - $7
Worth $1 per Collectible you have.
That information isn't hidden except with Masquerade. Again, a simple counter is enough. Add to the counter whenever you gain a Collectible, remove whenever you lose one. Masquerade is a potential hitch, but only with 3+ players. That's pretty easy to rule over though, either not affecting your Collectible counter or requiring you to disclose it. People would rarely be passing that card anyway.
Changing cards to a more individual level than what Travellers do would also be cool.
Chimera - Action 5c
Trash two Action cards from your hand.
This card's text and type are replaced by the union of the types and texts of those cards.
Maybe add a clause against Chimerizing Chimeras, or maybe not.
So it's supposed to permanently change Chimera? Again, individually label the Chimera cards. Have a token for each one. When you trigger the "merging" of the cards, set them aside together instead of trashing them and put the corresponding token on those cards.
Cards that operate differently based on your deck size or how many of X you have at any given moment are boons.
Those don't require a computer to track though.
It's not that hard to think of effects.
You could have effects on decks.
Like the attack Delayer: "Name a card. Each such card in opponents' decks that has a card with another name under it is swapped with that other card." (So it takes longer times for opponents to get the card you mentioned.)
You could have cards with mandatory negative reactions. Like a good action card but with the drawback that if someone plays an Attack / gains a Duchy / plays that very card / or something else... you have to discard it (or gain a Ruins, or ...)
Or other cards with forced actions that don't have to have a different backside because of it. Like an Action card that you have to play at the first opportunity.
You could have the game disclose some information that isn't normally available. Like an enhanced Mystic where you get to know the cost of your top card before trying to name it. Or a Spy that only tells you the cost of the spied cards. (Is that $8 card a Province or a Peddler?)
Or just actions that do different things depending on circumstances that don't have to be disclosed to opponents. (Something similar to Hunting Party or Menagerie without disclosing your hand.) It could be something that isn't disclosed to you either. Something Tribute-like that does things depending on the top card of opponent's deck without showing it. Maybe you don't even get to know what it did. You get a copy of the lowest card costing $3 to $6 in your left opponent's deck put at the bottom of your own deck. It could take a while until you see what it was. (This should be a new copy made, not taken from a pile.)
Cards that change other cards. The Silverizer that converts your topmost Copper in your deck (if there are any) into a Silver. Or the Enabler that changes all your Foos into Bars.
Etc. etc.
The conditions on Delayer are difficult enough that I don't think there's a way to do it IRL without being totally unreasonable, as opposed to just being somewhat unreasonable.
Mandatory negatives is good.
The partial knowledge thing like enhanced Mystic or nerfed Spy could sort of be done IRL if other players can be trusted to disclose that information honestly, but this is my favourite concept for this so far.
The undisclosed info stuff works well.
Silverizer can be implemented IRL. Play Silverizer, get a Silverizer token. Whenever you draw or reveal a Copper, you must return a Silverizer token if you have one to swap it for a Silver. This is not entirely enforceable, but trying to get around it means hiding a Copper in your hand instead of playing it that turn, lest your deception be revealed.
Enabler can be done similarly. However, this general idea can work if you make changes that aren't just card replacements, e.g. a Teacher that teaches a specific copy of a card.
How about a pile with infinite depth? It never runs out. Maybe the abilities and/or cost change as more cards are gained.
I proposed piles like that here but was thinking of a necessarily finite pile: http://forum.dominionstrategy.com/index.php?topic=15195.0
Surely you could make something that could scale arbitrarily high.
Infinite depth alone isn't enough -- note that treasure piles were originally meant to be infinite, but were made finite because
reality. So the IRL implementation would just give the pile ~40 copies and just deal with those rare times when the pile runs out.
But having copies of the card slowly change and scale infinitely as you go deeper into the pile, that works well.
Or, how about a card/event that gave a stash-like ability to another card - it could change its back color or something.
Have a bunch of attachable "tokens". To Stash-ify a card, attach a token to it. You'd need a good token design, of course. An actual clip would work, but it risks damaging the cards. Maybe a coloured card sleeve.
Easily Agitated Witch
$5 Action
+2 Cards
Each time your opponent moves his cursor over this card, he gains a Curse.
Indeed!
Edit: forgot to address the Dread Gazebo from the OP, but it's another one where you just need labelled cards and corresponding counters. It's obnoxious to handle IRL, but not impossible. Granted, Dread Gazebo does get a boost in that the actual choices you make on-play are extra annoying to track as X gets higher.