The egg cards have two purposes: they can increase their slot value for cards that don't rely on their own slot, and if you upgrade them they will "hatch" into strong cards.
Also please fix this bug:
Some observations/suggestions:
Edit: I also found a bug: If you win (or tie) a match and then press "Concede", it counts as both a win *and* a loss? I tied and pressed Concede and in my stats it said I had abandoned -1 games
Edit #2: I used 13 and X-stone on the final boss to obtain 323-stone, placed it on the board.. and apparently it doesn't work (17*19).
You can always add more mechanisms to allow these things. The "door" will end up becoming large and impractical after adding a few of these, so I see your point.
Here's an example of what I mean, using the I Wanna Lockpick Editor:
The 14(!) added doors incorporate positive real Master Keys into the design of the Exact Door.
(Also, it's still possible to make simple special Pure Doors, since they are immune to most effects.)
Actually, most of these that don't involve new key and door types are possible to make by filling a one-way passage with doors and keys (Refresh Key, Exact Door, Starry/Forceful/Weak objects, Colorless doors, possibly more that I didn't read and possibly Partial Blast Doors as well.)
It would definitely be nice to see these implemented though.
Possible bugs:
If the boss summons an ally and then is defeated, then the game will continue taking boss actions. If another enemy is summoned into the boss's location and defeated, the game will crash to the console and provide an error message about comparing nil to a number in function "enemyturn".
Summoned enemies in bossfight do not disappear after defeat.
After I defeated the boss, an extra boss turn was played before the "Victory!" prompt.
If SoulLeech is used on an enemy with different max HP and then you levelup at the end of the battle, then the new value will remain your max HP value.
A spell with a cooldown of 0 turns (+2 with Ice and -2 with Flame) is not unlocked immediately.
Boss's actions cannot backfire or fizzle.
I said "move to owner's discard pile" as a way to circumvent locked cards without complicated phrasing, because the text is long enough already. This would also use the interaction where locked cards that are moved somewhere other than the play pile remain locked.
If the ribbons were worth 2 instead of 3, then to create a blackjack with 3 ribbons each one could gain 5 value and then reset later. (In addition to being the value of the cards, 5 is also the value of the Akatan / Aotan combo.) I was actually thinking the different types of ribbons could have different functions. Poetry Ribbon could be another all-suits-at-once card, while Blue Ribbon could be spades or nothings and have an additional blackjack effect.
Rain Man (0):
On Play: Choose to add a Poetry Ribbon or a Blue Ribbon to your draw, play, and discard piles, then burn this card.
Ribbon (7):
On Play: If you have two ribbons of the same type already in play, move all other cards in play to owner's discard pile. You and opponent are each forced to stand, and your bust limit is set to 21.