I like echoes plenty, but I like seeing what can be done without adding new rules too. For my own amusement and to invite the fierce criticism of my peers, I'm making up my own fan expansion for brownbacked cards.
I am not 100% on how they should get included. I like the idea of dominionesque recombination of the cards, but lousy color streaks would be a pain, unless the cards are recombined by hand, which is tedious.
What I'm leaning towards the most is having a stack of 19 cards per age, and having the age be considered empty if 9 cards remain in it. That means returned cards are largely gone for good, so effects like Pottery can sift through poor color streaks without getting to the bottom of the stack and getting stuck with the same raw deal.
Theoretically, I'm trying to have the cards do a little bit of everything, just like the base set, so you could just play with the different set of brown backed cards all on its own. So there's always that.
I only have age 1 untested cards so far. Here are my beauties.
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AGE 1
Green
Lookout Post
[X]
[C][T][T]
T: I demand you reveal the highest card from your hand!
T: Reveal the lowest card from your hand. Draw a card of that value.
Ramp
[T]
[T][X][T]
T: Meld two cards from your hand.
Crop Rotation
[X]
[B][L][L]
L: If your score isn't zero, draw two cards of value equal to the number of different values in your score pile.
Yellow
Ingenuity
[B]
[X][T][B]
B: Return a card with a B from your board. Draw and meld a card of value 1 higher than the card you returned.
Storytelling
[C]
[L][X][L]
L: Tuck a card from your hand. Draw and score a card of value equal to the card above it.
Territorialism
X
[C][T][T]
T: I demand you meld the lowest card from you score pile!
T: Score the lowest card in your hand.
Red
Messengers
X
[T][C][T]
T: Draw and reveal two 1's. The player to your left chooses one. Keep that one, score the other.
Staff
[C]
[C]X[T]
C: Choose one of the six icons. You may not choose the same icon twice this dogma action.
Draw and reveal a [1]. If it has the chosen icon, then return it, you may splay your cards of its color left, and you may repeat this dogma effect.
C: If you chose all six icons, you win the game.
Fire
X
[T][T][T]
T: I demand you return a card from your hand!
Purple
Swimming
[X]
[L][T][T]
T: Transfer a top card from your board to the board of a player with no top card of its color. You may repeat this dogma effect.
T: I demand you transfer a top card of a color present on my board to my board if you received a card! Then I may splay that color left.
Latrine
[L]
[X][L][T]
L: Draw a [1]
L: You may return a card from your hand and draw a card of the same value.
You may return a card from your score pile. If you do, repeat this dogma effect.
Canoe
[X]
[C][L][C]
C: Draw and reveal a [1]. Meld it if its color is not present on your board. Otherwise, score it.
Blue
Scrolls
[X]
[C][B][B]
B: I demand you draw a [1]!
B: If any card was drawn due to the demand, draw a card of value one higher than the last card drawn due to the demand.
B: Meld the highest card in your hand.
Harvest Moon
[X]
[L][L][L]
L: I demand you score a card from your hand!
L: You may score a card from your hand.
L: Draw a card of value equal to the last card scored due to Harvest Moon.
Trading
[X]
[C][C][C]
C: Claim a standard achievement, if eligible.
C: You may return a top card from your board and draw two cards of value equal to the card returned.
C: You may return two cards of equal value from your hand and draw and meld a card of that value.
AGE 2
Blue
Modeling
[X]
[B][B][T]
B: Draw three cards, each of any value. Return all drawn cards with no [T].
Murals
[X]
[L][B][B]
[B]: Reveal any number of cards from your hand. Return one of them. Draw and score a card of value equal to the number of different icons revealed.
Red
Pulley
[X]
[T][T][T]
T: Draw a [2].
T: Meld a card from your hand.
Ram
[T]
[X][B][T]
T: I demand you draw a [3], then transfer the highest card from your hand to my hand!
Yellow
Assimilation
[C]
[L][X][C]
[C]: I demand you draw a [1]! If you have no [9] in hand, I may repeat this demand!
[C]: Draw and meld two [2]s.
Clans
[L]
[X][L][L]
[L]: Choose a value. Tuck that many cards of that value from your hand. If you do tuck that many cards, draw a card of value one higher than the cards you tucked.
Green
Tilling
[T]
[L][X][T]
[T]: Draw 3 [1]'s. Meld one, return the rest.
Taxes
[C]
[C][C]X][T]
[C]: I demand you transfer a [1] from your score pile to my hand!
[C]: Score a green card from your hand. Tuck a red card from your hand.
Purple
Great Walls
[T]
[T][X][T]
[T]: Choose a different color for every 2 [T] on your board.
[T]: Draw two [2]'s. Score each drawn card that matches a color you chose.
Archives
[B]
[X][B][B]
[B]: Choose a top card on your board. You may splay your cards of that color left and draw a card of that value.
AGE 3
Green
Trade Routes
[X]
[B][T][T]
T: Choose one or two top cards on your board and score them. If you scored two, transfer your bottom blue card to another player's board. If you do, that player transfers their bottom purple card to your board.
Castes
[X]
[L][L][T]
[L]: Tuck any number of differently valued cards from your hand. If you tucked at least four, repeat this dogma effect.
[L]: If you tucked at least one, you may splay your blue or purple cards left.
Yellow
Coliseum
[T]
[T][X][T]
T: You may return a card from your score pile.
T: Draw and score a card of value one less than the lowest card present in your score pile.
Monasteries
[B]
[B][X][L]
[B]: Draw a card of value 3 higher than your lowest top card
Red
Jesters
[B]
[B][X][B]
[B]: I demand you draw a card of any value and reveal it! Meld it or transfer it to my score pile, my choice!
Pharmacology
[X]
[T][C][T]
[T]: You may return a top card from your board.
[T]: Draw and tuck a [4].
Purple
Bards
[L]
[X][L][C]
[L]: Reveal a non-[10] card from your score pile. Splay your cards of its color such that non-top cards show as many icons as a third of its value, rounded up.
[L]: I demand you return a card from your score pile with a value less than the card I revealed!
Bloodlines
[T]
[X][T][T]
[T]: I demand you transfer a green card with no [C], a yellow card with no [L], or a red card with no [T] from your board to my board! If you do, draw a [1] and you may meld a card from your hand!
Blue
Curators
[X]
[B][C][C]
[C]: You may splay one color of your cards left. Draw and score a [2] for each newly revealed [T] revealed this way.
[C]: If you have no non-red top cards with a [T], claim the World achievement.
Relics
[X]
[C][C][C]
[C]: Draw and meld a card of value one higher than the number of claimed achievements.