Note: I'm archiving the old OP as it's been for the last several years, for posterity.
Over four years in the making (so far), here is the current form of my Enterprise expansion. It is designed to be a 300-card set, containing 25 Kingdom cards and at least one ancillary card. Right now it's in a bit of a state of flux, since I haven't gotten as much testing it as I would like to. This is what it looks like now, though, for some values of "now".
Its main theme is "effects with limited uses". This means there are a few one-shots and a few optional one-shots (à la Mining Village). The set also introduces Trade tokens, which you usually get when you gain a card and can use to boost that card when you play it. There are a few other cards that approach the concept of a one-shot in a creative way. Finally, there's a Treasures subtheme and a few off-theme cards.
The number of stars after each card name indicates how much testing the card has gotten. More black stars = more testing. Generally if a card has a bunch of stars, I'm happy with it or I would have scrapped it by now. Cards with more white stars need much more testing before I can make a judgment.
Kingdom CardsRedistrict: Action, $0 ★★★☆☆
Choose a card in your hand. Trash it and gain a card costing exactly $1 more than it. You may trash this. If you do, gain a card costing exactly $2 more than the chosen card.
Auction: Treasure–Reaction, $2 ★★★☆☆
+1 Buy
When you play this, discard your hand. Worth $1 per card discarded.
When another player plays an Attack card, you may discard this and another Treasure, to gain a Gold.
Jubilee: Action, $2 ★★★★☆
+2 Actions
+$2
You may pay a Trade token. If you don't, set this aside, and return it to the Supply at the start of Clean-up.
When you gain this, take a Trade token.
Wanderer: Action, $2 ★★☆☆☆
+4 Cards
The player to your left gains this.
When you buy this, put your deck into your discard pile.
Charlatan: Action–Attack–Duration, $3 ★★☆☆☆
+1 Action
Now and at the start of your next turn, +$1. Until your next turn, each other player must use all of their Buys during their Buy phase.
Convoy: Action, $3 ★★☆☆☆
+3 Cards
Discard a card. You may pay a Trade token, to play this again.
When you gain this, take a Trade token.
Floodgate: Victory, $3 ★★★★☆
Worth 2 VP.
When you gain this, aside up to 4 cards from your hand. At the start of your next turn, put them into your hand.
Gambler: Action, $3 ★★★★★
+1 Card
+1 Action
Look at the top card of your deck and choose one: Trash that card; or put that card into your hand and trash this.
Tinker: Action, $3 ★★★☆☆
+$1
Trash 2 cards from your hand. You may pay a Trade token, to gain a card costing up to their total cost.
When you gain this, take a Trade token.
Dignitary: Action–Reaction, $4 ★★★☆☆
+2 Cards
+$2
Put 2 cards from your hand onto your deck.
When another player plays an Attack card, you may discard this, to trash a card from your hand, then draw 2 cards.
Profiteer: Action, $4 ★★☆☆☆
Gain a Gold onto your deck. Each other player gains an Armament from the Armament pile.
Stockpile: Treasure, $4 ★★★☆☆
Worth $1
When you play this, you may pay a Trade token, to gain a copy of a card you have in play. Otherwise, take a Trade token.
Terrace: Action, $4 ★★★★☆
+1 Card
+2 Actions
You may pay a Trade token, to discard your hand and draw 5 Cards.
When you gain this, take a Trade token.
Almoner: Action, $5 ★★★★☆
Discard any number of cards, then draw until you have 7 cards in hand. Each other player may gain a Copper to their hand.
Bladesmith: Action, $5 ★★★☆☆
Gain an Armament from the Armament pile to your hand.
Conclave: Action, $5 ★★★☆☆
+1 Action
Reveal the top 3 cards of your deck. From those cards, put an Action card, a Treasure, and a Victory card into your hand. Put the rest back in any order.
Fund: Treasure, $5 ★★★☆☆
Worth $2
+1 Buy
When you play this, you may trash it, to gain a Silver to your hand.
General: Action, $5 ★★★☆☆
You may play an Action card from your hand twice. When you discard that card from play, you may put it onto your deck.
Harbor: Action, $5 ★★★☆☆
Look at the top 3 cards of your deck. Put any number of them into your hand. Put the rest back in any order. +$1 per card you put back.
Vendor: Action, $5 ★★☆☆☆
+1 Card
+1 Action
+$1
Look through your discard pile. You may pay a token, to put a card from it into your hand.
When you gain this, take a Trade token.
Study: Action–Duration, $6 ★☆☆☆☆
+$3
During your next turn, when you play an Action card, first draw until you have 5 cards in hand.
Ancillary (Non-Kingdom) CardsArmament: Treasure–Attack, $4* ★★★☆☆
Worth $2
When you play this, if you have another Attack card in play, return this to the Armament pile and each other player gains a Curse.
(This is not in the Supply.)
OuttakesThese cards don't fit into Enterprise anymore, but most of them were balanced and all had their fans. A version of Conclave was cut for balanced reasons, and the tweaked version is here since I haven't tested it myself yet.
Mill Town: Action, $3 ★★★★☆
+1 Card
+2 Actions
Discard a card. You may reveal your hand, to gain a card costing up to $1 per Copper in your hand.
Refurbish: Action, $4 ★★★☆☆
+1 Buy
You may trash a card from your hand to gain a Silver.
While this is in play, Silver produces an extra $1.
Barter: Action, $5 ★★★☆☆
+1 Action
Trash a card from your hand. Gain a card costing up to $2 more than it. You may pay a Trade token to put the gained card into your hand.
When you buy this, take a Trade token per Silver you have in play.
Phew, that's all the cards. Again, I'd like to thank rinkworks for his encouragement during this set's first few baby steps. My, how far we've come. I'd like to thank lympi for helping me learn how to mock up card images. I'd especially like to thank those of you who have playtested my cards and given me feedback. This set would not be as good as it is today without you! I will continue to strive to improve it.