A few additional thoughts on what qualifies as recursion (and an answer to LastFootnote's question):
Do cards that can replay themselves without moving qualify?
For example:
In the spirit of not disqualifying cards, I'll allow cards that replay themselves.
However, I think the heart of what recursion means is that a card can be played again and again, potentially an unlimited number of times. Once you've played Tent, Merchant Camp, or Conjuror, you could play them again every turn for the rest of the game.
That looks very different than a card that can be played twice (or even multiple times--I don't know exactly how this mechanic works) in one turn by replaying itself. In the latter case, I would think of the multiple plays of the card as a scaling power of the card's ability, more akin to
Shepherd's ability to draw more cards depending on how many Victory cards you discard.
That being said, one could imagine, for example, a Duration card that said something like:
+2 Cards
At the start of your next turn, +1 Buy, and you may discard 2 cards to replay this.
Something like that would very clearly fit into the idea of recursion, and be very similar to the existing recursion cards. There may be other ways of making a card endlessly replayable without moving it, which I will leave up to you all to come up with.
Since I am on the topic of what I consider recursion, I will preemptively add that I would not consider
Scheme to be a good example of a recursive card (although, again, definitely would not disqualify it). In the vast majority of cases, the point of Scheme is to topdeck some other Action card that you want to replay sooner, or to topdeck Scheme so it is there to actually be useful when you need it. As a cantrip, there is usually no point to playing Scheme itself, except to let you replay another card. That being said, a card with the ability of Scheme that would more-often than not be used to topdeck itself would definitely count.
Also, one more design tip. Don't forget the existence of
Champion and the
+1 Action Token from Adventures. If your card says "+$2, return this to your hand" then, combined with one of the above, quickly and easily produces and unlimited amount of $. Under the category of "balance" I'll consider anything that is completely busted when paired with any official card/landscape to be completely busted.
Edit: Having tracked down the
original post explaining how Activation cards work, I would re-state my position that, while not disqualified, this is outside of what I would consider to be recursion. As I understand the mechanic, a player could double-play this card each time they play it, as they will get a Gem on-gain and another one after every play when they Activate the card. So double-playing it would be the default, but a player could opt to single play it once in order to save up gems to get multiple plays in the future.
However, I think the Activation ability has great potential in this contest, as an Activation that moved the card into a player's hand or onto their deck (or even an Activation that replayed the card, but that would have Champion/+AT issues) could be great examples of recursion.