Here is what I came up with:
Arctic Castle - $4
Action - Victory - Arctic
Each player reveals their hand, then gains a Snow for every 2 cards revealed that are not Snow (rounded down).
---------------
Worth the greater of 2VP or 1VP per 3 Snow cards you have (round down).
---------------
When you buy this, you may discard an Action card to gain a card from the Snow Gear pile. If you discarded Snow, +1VP.
The card comes with a Snow Gear pile, full of items useful for your Arctic expansion. There is one copy of each card in the pile, which can be gained either through Arctic Castle's on-buy mechanism or Winter Vendor, if anyone gains and uses it. There are 16 cards in the pile:
Snow Gear Pile:
Elegant Muff - $2*
Treasure - Snow Gear
$1
You may put up to 2 cards from your hand onto your deck.
(This is not in the Supply.)
Snow Shoes - $2*
Treasure - Reaction - Snow Gear
$1
---------------
When you play Snow, you may first reveal this from your hand once per Snow played for +1 Card.
(This is not in the Supply.)
Brandy - $3*
Treasure - Snow Gear
$2
+1 Buy
(This is not in the Supply.)
Snow Shovels - $3*
Action - Snow Gear
+1 Action
Do the following any number of times: discard a card from your hand; if it was a Snow, draw a card.
(This is not in the Supply.)
Winter Vendor - $3*
Action - Snow Gear
+1 Card
+1 Action
Each player (starting with you) may discard any number of Treasure cards from hand to gain a card from the Snow Gear pile worth up to $1 per card discarded.
(This is not in the Supply.)
Brazier - $4*
Treasure - Snow Gear
$2
You may return up to 2 Snow cards from your hand to their pile.
(This is not in the Supply.)
Sled - $4*
Action - Snow Gear
+1 Action
Reveal cards from your deck until revealing one that isn’t Snow. Put all of those revealed cards into your hand.
(This is not in the Supply.)
Snowman - $4*
Action - Snow Gear
+1 Card
+1 Action
Each player may choose to reveal a hand with at least 3 Snows in it for +3 Coffers.
(This is not in the Supply.)
St. Bernard - $4*
Action - Reaction - Snow Gear
+2 Cards
---------------
When another player plays a Snow, you may first reveal this and a Snow from your hand; if you do, play this.
(This is not in the Supply.)
Snow Golem - P$4*
Action - Snow Gear
Reveal cards from your deck until you reveal more non-Snow Action cards (other than Golems) than Snows. Discard the Golems and non-Action cards, then play the non-Snow Action cards in any order. Place half the Snows (rounded up) onto your deck, and discard the rest.
(This is not in the Supply.)
Four-Poster Bed - $5*
Treasure - Night - Snow Gear
$2
If it's your Night phase, reveal and set aside a Victory card and an Action card costing less than it from your hand, and put both into your hand at end of turn (after drawing).
(This is not in the Supply.)
Golden Goose - $5*
Action - Reaction - Snow Gear
Gain a Gold.
---------------
When you gain a Snow, if it isn't your turn, you may play this from your hand.
(This is not in the Supply.)
Ice Magician's Cloak - $5*
Action - Snow Gear
For the rest of your turn when you play a Snow, get +1 Card and +1 Action instead of following its instructions.
(This is not in the Supply.)
Snow House - $5*
Action - Snow Gear
+$5
-$1 per 5 cards in the Snow pile (rounded down).
(This is not in the Supply.)
Lead Glass Windows - $6*
Action - Duration - Snow Gear
While this is in play, when you play Snow, add 1VP to this. At the beginning of each of your turns, remove 1% from this. When you buy a Victory card, take the VP from this and discard this from play.
(This is not in the Supply.)
Regal Cloak - $7*
Action - Treasure - Snow Gear
+2 Actions
For the rest of the turn, when you gain a Snow, +1 Villager.
$3
(This is not in the Supply.)