I've now playtested and tweaked cards in this thread, and I thought I'd post my findings.
Investment
$5 - Treasure
Worth $2. Worth an additional $1 per empty supply pile.
Definitely strong, arguably too strong, but too weak at $6. Best in Colony games.
Mad Scientist
$3 - Action
+1 Card, +1 Action. For each empty supply pile, +1 Card.
Stunningly broken at any price. I gave up on this one. It's actually just kind of boring. Fully powered up, there's no longer any gameplay: you just lay them all down, and do what you will with the rest of your deck. Greening doesn't particularly slow them down, so those strategic considerations are non-issues. Fully powered Cities, by contrast, don't necessarily draw your whole deck and allow you to make decisions about what to do with the extra actions.
Burglar
$5 - Action/Attack
+3 Cards. For each empty supply pile, +1 Card.
With no empty supply piles, each other player having 5 or more cards discards his hand and draws 5 cards.
With 1 empty supply pile, each other player having 4 or more cards discards his hand and draws 4 cards.
With 2 or more empty supply piles, each other player having 3 or more cards discards his hand and draws 3 cards.
This is also broken, I suppose because of how wildly different the power of the attack is. At level 1, it's scarcely an attack at all. At level 3, it's brokenly powerful. At level 2, it just does what another card already does.
I recognized early on that the drawing component of the card exaggerated the unbalancing, so I ripped that out and tried a few different flat bonuses. But I soon discovered that the attack component was unfixable. Ripping that out and restoring the variable drawing piece, I was somewhat surprised to find, works just fine:
Gatherer
$5 - Action
+3 Cards. For each empty supply pile, +1 Card.
Sorceress
$5 - Attack
Each other player gains a curse.
If 1 or more supply piles are empty, +2 Cards, +1 Action.
If 2 or more supply piles are empty, +$1.
I figured the coin was too strong, so I changed it to +1 Buy before even playtesting it. After a single game, it was clear it was still grotesquely powerful, so I priced it at $6. But this was the wrong solution. What happens is that Sorceress is still a must-buy at $6, so players just dive into the pile, suffering through the collisions. The Sorceress pile emptied first -- I took the last one, quite apprehensively, as it meant my opponent would get the benefit of the cursing draw engine before me. But I knew I'd have three Sorceresses in my next hand, so I took the risk. My opponent handed out two Curses, and then I handed out the last three.
After a little more thought, it was clear that the drawing engine piece had to be put at Level 3, rather than Level 2. That enabled me to drop the price back to $5, where it belongs. That allowed me to throw away the irrelevant +Buy bonus, but I'm still deciding what to put at Level 2. My current thought is one of these:
Sorceress #1
$5 - Action/Attack
Each other player gains a Curse.
If one or more supply piles are empty, +2 Cards.
If two or more supply piles are empty, +1 Action.
Sorceress #2
$5 - Action/Attack
Each other player gains a Curse.
If one or more supply piles are empty, +1 Card, +1 Action.
If two or more supply piles are empty, +1 Card.
I'm pretty sure both of these ideas are improvements; it's just a matter of figuring out which one works best. Both retain the interesting decisions Sorceress players have to make about when they empty a pile, whether it's the Sorceress pile, Curse pile, or some other spammable pile.