The expansion will either come with a new, big box version of alchemy, or be a small expansion of 150 cards that you can buy after Alchemy.
New Mechanics:
Rack: Cards on your rack go into your hand at the start of your turn. There are many ways to get cards on your rack in this expansion. "Rack a card" means "put the top of your deck" (ie like drawing except instead of putting it in your hand, put it on your Rack), but a card may also ask you to rack a specific card (eg "Gain a Silver and rack it").
Dual cost: Cards and events in this expansion may have 2 costs. If either of the costs has P, include potion in the Kingdom.
Both costs are "present". If either of the costs satisfy a condition you must count the card, so if you are required to gain a card with one of the costs (eg upgrade), you must gain it.
If a task cares about the cost of a card (eg gaining a card based on the cost of the card you trashed with Remodel), the player doing the task in question chooses which of the two costs to work with.
You may buy the card by paying either of the listed costs
However you may choose either cost for the card you just bought when referring to any cards that care about the cost of a card you just bought (eg Talisman, Haggler)
New tokens with replacement randomisers:
Like Adventures, New Alchemy has tokens that can be put on cards. Events may refer to these tokens. Additionally, some cards from the old Alchemy expansion have new randomisers that refer to the placement of these cards.
They are
Transmute: "Place every player's 2 Buys token on this card (when you gain a copy of this card, +2 Buys)"
Scrying Pool: "Place every player's block token on this card (when another player plays this attack card, it doesn't affect you)"
Familiar: "Place every player's -$1 cost token on this card" (costs $1 less but not less than 0)
Philosopher's Stone: "Place every player's Rack token on this card" (when you gain this card, rack it)
Warlock
Action/Attack - $3P/$4
+$2
+$P
Each other player puts a potion from their hand on their deck (or reveals a hand with no Potion)
---
When you gain this, each other player gains a Potion.
Plenipotentiary
Action/Attack - $4P/$6
+4 Cards
You may return a Potion to the supply from your hand. If you do, each other player gains a Potion and a Curse.
Diviner
Action - $P/$3
+2 Actions
+$1
Reveal the top card of your deck. If it has $P in its cost, put it in your hand. Otherwise, rack it.
Demystify
Action - $2P/$5
Trash a card from your hand. If it has P in its cost, +1 Action. Gain a card costing up to $2P more than the trashed card.
Element Collection
Victory - $P/$4
Worth 1VP for every X Potions you have in your deck, rounded down (where X is the number of cards in the kingdom with Potion in its cost)
Brewery
Action - $P/$4
+1 Card
+2 Actions
You may discard a Potion; if you do, +1 Card, +2 Actions
---
When you gain this, +1 Buy
Summoner
Action - $5
Rack this
---
While this is on your rack, when you gain a card, rack it
(4 cards to add)
Events
Course
Event - $P
Trash a card you have in play. Gain an Action card costing up to $1 more than it.
Reflection
Event - $P
+1 Buy
Rack a card you have in play
Conjure
Event - $P
+1 Buy
Gain a card costing up to $4
Bronze Worker
Event - $P
+1 Buy
Rack a card for each Copper you have in play
Tactic
Event - $P
+1 Card
Rack your hand
Melody
Event - $P
+1 Buy
Reveal the top 3 cards of your deck. Rack the Actions, put back the Treasures in any order, and discard the rest.
Display
Event - $1P
+1 Buy
Rack a card from your hand. +$1 for each card on your Rack.
Fortify
Event - $P/$2
Once per turn: +1 Buy. You may take back your Block token from a card. If you did, +$4. Otherwise, place your Block token on any supply pile (attacks with the Block token don't affect you when played by others)
Distribute
Event - $P
+2 Buys
Once per turn: Place your -$1 Cost token on any card (this card costs $1 less, but not less than $0)
Rift
Event - $P/$2
+1 Buy
Place your Rack token on an Action supply pile (when you gain a card from that pile, rack it)
Circus
Event - $P/$3
+1 Buy
Place the +2 Buys token on a card costing at least what you paid for this event (when you gain a copy of that card, +2 Buys)
Gale
Event - $P
+1 Buy
Rack 2 cards. Each other player discards their Rack or the same number of cards from their hand.
Mirror
Event - $1P
+1 Buy
Trash a card you have in play. If you did, play a Treasure you have in play again.
Trainee
Event - $1P
+1 Buy
Trash a card from your hand. Rack a card per $ it costs. +$2 if it had P in its cost.
Sneak
Event - $P/$2
Trash a Potion you have in play. If you did, +2 Actions, +2 Buys, Gain a Silver and return to your Action phase.
Event - $P/$5
+1 Buy
Gain a Gold, a Potion and a Copper
Grand Design
Event - $8P
Trash a Potion from play. If you did, at the end of your turn, rack your deck and discard pile.
Cocktail
Event - $PP
+2 Buys
+$P
+$3
Gain a card costing up to $6
(6 Events to add)
Clarfications:
Dual Cost Cards:
You can gain a card with a dual cost with Smugglers even if the player to your left paid more than $6 for it (similar to Bridge, Highway)
If a dual cost card is trashed with Swindler, the player who played the Swindler decides which of the two costs they wish to use for the card's effect (as the card specifies "you choose") but a copy of the card they just trashed is always available as one of the options to replace the trashed card (as long as it's in the supply).
Saboteur and Sage will stop revealing cards if it reveals a dual cost card with a cost higher than $3. With Saboteur the player can gain a card costing up to $2 less than either cost.
If the card is revealed with Knights or Rogue and one of the costs is between $3 and $6, it is one of the cards the player is able to trash (falling under the "both costs are present" rule), and they must trash it if the other revealed card does not have a cost between $3 and $6. Similarly Rogue gains it if it's in the trash and you can't choose to attack instead if it's the only card costing between $3 and $6 in the trash.
Giant and Warrior are asking the player being attacked what the cost of the revealed card is (as it's in the "each other player" portion of the card), so they get to choose what happens.
The player who played Chariot Race chooses the cost of all the revealed cards
Fortify: You can place the Block token on any card, but it only cancels effects for Attacks played by others. You will still get the effects of cards like Council Room and Masquerade, and you can still play attacks.
Brewery/Circus: +buy has no effect if it isn't your turn.