Here are a few more ideas I came up with and didn't get to playtest, yet. I tend to miss some important flaws when designing cards so I'll take any feedback into account before mocking these up.
Building Crane X, $3<2>, Action
+3 Cards. +1 Action. Return this to the Supply. Building Cranes cost $3 less this turn, but not less than $0<2>.
Building Crane costs $5 and when you return it, it costs $2 less. Depending on the +Buy situation in the Kingdom, the power of this effect ranged from non-relevant to absurd. Sometimes you could just drain the BC pile (costing $0) with tons of buys and set up a megaturn. I'm considering giving BC a coin-debt-hybrid cost so that you can still drain the pile with buys but you can never get them for free. You'd always have to pay at least <2> per Crane on your next turn. If that's too cheap I could adjust it to <3>. How do you like it? Can the wording on BCX be better?
Smeltery, $4, Action
Trash up to 3 cards from your hand. Return up to 3 cards with the same name from the trash to the Supply. +$1 per card you returned.
Games using this can’t end on 3 empty piles unless the trash is empty.
I had this idea for a bottom part (which probably countless people had before) and came up with a nifty top to work with it. Either it leads to interesting games, or it’s mostly annoying, we’ll see. It might also be too strong.
Gem, $4, Action/ Treasure
If it’s your Action phase, +2 Cards. If it’s your Buy phase, +$2.
While this is in play, if you played exactly 4 cards, return this to your hand.
Inspired by Asper’s Jeweller (which didn’t work out) and Road, this card is very weak when played once but if you put some work into it, can boost your turn multiple times. Alternatively, it could go back to your hand on different triggers depending on your current phase (e.g. 2 actions in your Action phase and 3 Coppers in your Buy phase).
Recruiter, $4, Action/ Looter/ Gathering
+2 Actions. You may gain a card costing up to 5. If you do, return this to the Supply and add 1 VP to the Recruiter/Crusade Supply pile.
When you buy this, if there are any VP on the Recruiter/Crusade Supply pile, take them, instead of gaining this.
Split pile
Crusade, $5, Action/ Attack/ Looter/ Gathering
+2 Cards. Each other player gains a Ruins. Add 2 VP to the Recruiter/Crusade Supply pile.
I wanted another Gathering pile like Temple from which you take the VP on-gain but of course different from Temple in all other ways. It’s got the following premises (i.e. things I want in its concept):
• It’s a Gathering split pile
• The top card returns to the Supply to block the bottom card
• The bottom card adds more VP to the pile than the top card
• One of the two cards can gain cards to circumvent the VP-taking (in case you’d rather have the card)
So far, I’m not very pleased with the whole thing. It seems clunky but perhaps you can see what I’m going for. I’ll keep thinking about it.
Bootlegger 2, $3, Action
+2 Cards. +1 Buy.
Setup: Add an extra Kingdom card pile costing up to $2 to the Supply. When you gain a 2nd card in one of your turns, also gain a card from that pile. When you trash a card from that pile, return it to the Supply.
Just another Supply-related thing I want to try out. It floods your deck with a cheap card you may or may not want. Big Money resists it. I have no idea if that’s actually an intriguing concept.