Cottage
$2
Action - Duration
+2 Actions, +1 Card. While ~ is in play, at the beginning of your opponents' turns, they get +1 Action.
This is awful. I would NEVER use this. If there's a decent terminal on the board, then I'm wasting a turn buying something that will help my opponents as much as me. A $2 should be almost as good as a $3 and a vanilla village is much better than this. Brainstorming here, but maybe (changes in bold):
Cottage
$2
Action - Duration
+2 Actions, +1 Card.
When you buy this, +1 buy.
If this is in play and you have more than 1 action point at the start of Clean-up, at the beginning of your opponents' turns, they get +1 Action.
(so, you can build as many as you have the money to. And if you actually need to use the cottage for something, then your opponents don't benefit, but if there's space available, the magic of friendship compels you to share. The wording might need some tweaking to handle the possibility of multiple cottages in play. And it still seems to weak if it's a duration - it will then miss reshuffles. So a wordsmithing that will let it go into the discard at clean-up is also a good idea.)
Zecora
$2
Action
+1 Action. Each player may trash a card from their hand. For each player that does, draw a card.
I like this, although running thru the scenarios it's a bit too good. To take the scaling out of it, how about:
Zecora
$2
Action
+1 Action. Each player may trash a card from their hand. If you do, +1 card. If any opponent does, +1 card.
(Non-exclusive, so this could give you 2 cards at most)
This is a bit weaker than how you had it, but if you think about it.. if only you trash, then this is a trashing cantrip, which is like a weaker Upgrade - strong for $2. If you and your opponent(s) trash, then this is a lab that also gives everyone an equal benefit - GREAT for $2. If only your opponent(s) trashes, then this is a cantrip that helps them - lousy. And if nobody trashes than this is a dead card like most late-game trash cards. Overall the effect for $2 is balanced I think.
Dragon
$4
Action - Attack
+2 Cards
Each opponent discards a Gold or reveals a hand with no Gold.
+2 Cards is similar to +$2 in power, so offhand this looks like a strictly better cutpurse.. except that you have no choice but to use copper, but you do have a choice whether to use Gold.
If you want to hit platinums too without awkward wording, try:
Dragon
$4
Action - Attack
+2 Cards
Each opponent discards a treasure with a cost of at least $6 or reveals a hand with no such treasures.
It would be like a dragon to keep harems, too..
Pinkie Pie
$4
Action - Duration
+2 Actions, +1 Buy, +$2
At the the start of each opponent's turns until your next turn, they get +1 Action, +1 Buy
Man, this is the same issue as Cottage, as it "nets" you only +1 action, +$2 which is roughly the same as silver. So reduce the cost to $3 maybe?
Princess Celestia
$4
Action
You may choose an action card in your hand. Play it three times.
Return ~ to the supply. Each other player gains a copy of Princess Celestia.
Again seems to benefit opponents way too much. Playing something three times is nice but carries a huge cost in this case. Maybe:
Princess Celestia
$4
Action
You may choose an action card in your hand. Play it three times.
Each other player gains a copy of the played card.
Princess Celestia masters something, then teaches her subjects how to do it. But she's still better at it.
Rarity
$4
Action - Attack
Choose one: +$3, each opponent gains a gold, or +$2, each opponent gains a silver; or +$1, each opponent gains a copper.
I don't think this is too powerful. Coppers are way less bad than curses, and you're hurting your own potential by $1 and an action point by using her to spread coppers instead of having bought a silver.
Applejack
$4
Action
+2 Cards, +2 Actions, +1 Buy
Each other player gets +1 VP.
I like this a lot. Without the enemy-benefit this would be a strong $6 or a weak $7. I think it's balanced.