c1 - Treasure - 5$
Play this as if it were a Treasure card in Play that you choose. This is that card until it leaves play.
The card that eventually became Counterfeit started out in Prosperity as just a Throne Room for Treasures. That's basically what this is except you can use any number of these to copy the same Treasure, which is nice. I think probably it should cost less. Like, $3. There's no guarantee it'll be worth more than $1, or even if it'll be worth anything at all.
The swinginess problem (especially with Platinum) may or may not be an issue. It's worth noting that Counterfeit solved it by making you trash the Treasure, making it better when it was at its worst (Copper) and worse when it was at its best (Platinum). Personally, I'm not sure this is different enough from Counterfeit to be interesting.
c2 - Action - ??
+1 Action
Look at the top X cards of your deck. Put one of them into your hand and discard the rest.
Donald tried so many versions of this. The ones in the Outtakes article all looked at 4 or more cards. It's possible that a version that looks at 3 cards could work. That feels useful, but not crazy. Probably such a card would be cheap (≤$4). Maybe it should put one of the other cards back on your deck? Don't know why, just a thought I had.
c3 - Action - 2$
+1 Card
+1 Action
You may put your deck into your discard pile.
The problem here is one of logistics. Cantrip Chancellor is just begging you to reshuffle several times during your turn. Play c3, put your deck into your discard pile. Play another one, shuffle, draw a card, put your deck into your discard pile. Play Smithy, shuffle, draw 3 cards.
I could maybe see a version that put your deck into your discard pile at the end of your Buy phase. Then I thought, having more than one of them is now moot, so maybe it should have some vanilla bonuses that weren't so spammable. Something like [+1 Action; +$2] at $5. Then I realized I had invented an (arguably worse) version of Royal Seal.
c4 - Action - ?
+1 Action
Choose one: +1 Card; or look through your discard pile, and put a card from it into your hand.
Eh, pretty similar to Scheme, but probably stronger in most cases? I don't really love the idea.
c6 - Action - 5$
Reveal your hand. +1 Card per differently priced card in it that's worth between 2$ and 6$
Doesn't really excite me. I'd try it without the cost bracket. Just, +1 Card per differently costed card in your hand. See how that works before nerfing it.
c7 - Action - 5$
Reveal your hand. +1$ per non-Treasure card in it.
Seems very similar to Secret Chamber. Except it stacks, and you can still play those Action cards afterward. But you need a Village for that. Well, it doesn't excite me, but it might work.
c8 - Action - 4$
+4$
+1 buy
Each other player may trash 2 cards from his hand and gain a card costing less than both of them combined or a curse.
How about, instead of a Curse, it's just "but not less than $0" so that they can gain a Copper instead? The Curse is just a kludgy way of saying, "Hey, you can't just trash 2 cards with this," but it still looks bad. Trashing two Coppers from hand to gain a Copper into your discard is still plenty weak.