Here's some of the thinking that went into the card.
version 1
Stitched-Together Monster
Cost: Cost of Monster Card 1 + Monster Card 2
Type: Types on Monster Card 1 and Monster Card 2
This card is the two Monster cards with the type, text, and cost combined.
When you play this, play as if each Monster card in either order and each other player gains 2 VP tokens.
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Setup: Select two cards not in the current kingdom. Those cards are the Monster cards.
The basic idea is simple. It's two cards in one - the cost, text, actions, everything and you get to play both cards in a single play. (Inspired by jotheonah CopperSmithy in the Bad Card Ideas thread).
And irrevocably broken. Overpowered, over expensive (unless way under priced like Secret Chamber/Poor House). I made a feeble attempt to nerf it by giving the other players victory points on play, but that feels off theme and pasted on. And usually the card would be too expensive. By the time you could reliably afford it, you'd be in the end game. I gave up on it for a while.
version 2
Stitched-Together Monster
Cost: $3
Type: Action*
Choose one:
Return this card to the supply. If you did, add a card from your hand to your Monster Mat.
--or--
Play this as if were the cards on your Monster Mat in any order. This is those cards until it leaves play.
This card shares the type of the cards on your Monster Mat. Cards on Monster Mats are returned to your deck at the end of the game.
Now this is better. The card is functionally still expensive, but now the cost is spread out. To build your monster, you have to buy parts, and keep rebuying monsters. I like the mini-game of competing for monsters. And feels thematic that you could loose control of the monster.
But now it's more like a super Band of Misfits. Instead of being the cards at all times, it's just the sum of its parts while in play.
And what if you put a Stitched-Together Monster on your Monster Mat? Can't let that happen.
version 3
Stitched-Together Monster
Cost: $3
Type: Action*
If there are no Stitched-Together Monsters on your Monster Mat, this is the cards on your Monster Mat and share their types and abilities. You may put this card on your Monster Mat.
Otherwise, return Stitched-Together Monsters on the Monster Mat to the supply, and if you did, set aside a card from your hand face up on your Monster Mat.
Cards on Monster Mats are returned to your deck at the end of the game.
Aagh! Too many words. Solves the problem with putting Stitched-Together Monster on the Monster Mat, but can be better. Maybe splitting it in two.
version 4
Mad Scientist Lab
Cost: $2
Type: Event
Put a card other than Stitched-Together Monster from your hand face up on your Monster Mat.
Cards on Monster Mats are not returned to the owner's deck at the end of the game.
Stitched-Together Monster
Cost: $2 Potion
Type: *
This is the cards on your Monster Mat and share their types and abilities.
Setup: Add Mad Scientist Lab Event.
Much better. At a point where it could be play-tested. Adding a potion cost to Stitched-Together Monster means I can't indirectly put one on the Monster Mat via Band of Misfits or Inheritance. It also delays how fast you can play it. Thematically, it feels like an Alchemy card. (To be honest, I don't feel potions get enough love.)
On the principle of testing the simplest card possible, I removed the requirement to return the Stitched-Together Monster to the supply. I'm tempted to require a Stitched-Together Monster in play for the Mad Scientist Lab. Currently it can be used independent of the Stitched-Together Monster as a highly inefficient trasher. I changed it so the cards are not returned to the decks at the end of the game - putting Provinces on the Monster Mat seemed too powerful otherwise.
I still need to tweak the pricing, and maybe a better name for the event. Starting with a cheap price, just to see how it works out.