So I've been thinking about some ideas in my head for some time, and I think I am ready to release these ideas. I haven't tested any of these, and it will be fun to do that at some point. Anyways, I have no idea what to do for the theme, but maybe it'll be Greek or something. Anyways, on to the cards.
Firstly, the most drastic idea, which has been explored in a lot of ways but not like this to my knowledge. It's the idea of replacing the starting base cards with something else, which comes with a bunch of logistical problems, but I think I have found an interesting way to make this work.
So each of the Base Starting cards has two potential replacements. This means Curse, Copper, Silver, Gold, Estate, Duchy, and Province. I've decided that these things would have to be public, so readily available to each player. Of course, replacing all of the cards multiple times over is a giant hassle, so I've decided on the idea of single randomizers instead. This way, it's small and you don't have to use a different pile for different base cards, you just ghost it instead, pretending the Copper is that public card. The only potential issue is that people might forget.
So the way this works is that zero to three of the base cards change ideally per game, that would be my initial recommendation. There will be a placement card that is public that will 'replace' the base card, so you would buy for example Copper, but the Copper would be treated like the replacement card. In my mind, this replacement card would still be considered Copper, so there wouldn't be any annoying issues with say, Moneylender if you wanted to use it. I'll probably have the header text read as "Copper [Ossified]" so it's both distinct but also considered a Copper at the end of the day.
Anyways, on to the card ideas. These are raw ideas, but they will be fun to test!
Coppers
Copper [Ossify] Action-Treasure $1
+$1
+1 Action
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$1
Basically, Coppers become Action cards. You can Throne them now for Actions, or to help with Library in multiple ways, or make Necromancer more interesting, for example. You can even play them as Treasure cards instead, there's that flexibility. that Copper turns into an Action card kind of reminds me of turning into Stone, hence the name, and there's a potential Medusa/Gorgon theme there.
Copper [Unnamed] Action-Treasure $0
+$2
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$1
Terminal Silver. This one might change, it's not too hot.
Silvers
Silver [Unnamed] Action-Treasure $3
+1 Action
You may discard a card, to draw +1 Card.
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$2
it's a weak sifter but it provides additional options over simply being a Silver. Maybe you really want to cycle and draw something potentially nice.
Silver [Unnamed] Treasure $3
$2
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When you trash this, +$1.
Nice for remodeling and trashing.
Golds
Gold [Cursed] Curse-Treasure $5
$3
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-2 VP
So yeah, it's cheaper but it's also Cursed. It runs into problems with Cursers though so I think this one won't make it. Kind of a filler idea.
Gold [Unnamed] Action-Treasure $6
You may trash this, to gain a card costing up to $5.
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$3
Duchy gainer in games, Gold gainers become even more enginey, and there's this cute flexibility going on here.
Estates
Estate [Sunken] Victory-Reaction $2
1 VP
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When you discard this other than during Clean-Up, +$1.
Cute with sifters and Baron. Maybe exploitable though. I also don't have a second Estate idea at the moment.
Duchies
Duchy [Royal] Action-Victory $6
You may play an Action card from your hand twice.
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2 VP
Throne Room Duchy. Engines go bananas. I thought about a +buy Duchy but I think this is potentially more intriguing. I'm afraid to put it at $5 but it might be fine.
Duchy [unnamed] Victory $6
3 VP
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If this pile is empty at the end of your turn, the game immediately ends.
It's another end game condition! Might enable some cool rushes or alternate strategies, and it costs $6 so it's harder to do.
Provinces
Province [Unnnamed] Victory $7
5 VP
Just more potential timing to think about, plus remodeling changes, etc. I thought about doing a gambit Province where you can play it to trash it, then draw 5 extra cards on your next turn but that sounds really stupid. It also sounds really fun though. No idea what to do for the second slot.
Curses
Curse [Doomed] Action-Curse $0
Trash this, to gain 2 Coppers.
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-2 VP
More junk but at least it's not -2 VP!
Curse [unnmaed] Action-Curse $0
+1 Card
+1 Action
If there are two Curses in play (including this), go directly into the Buy Phase.
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-1 VP
You can sift with these Curses, but only a little bit! Maybe it can be upped to 3 Curses in play, it just needs actual testing. It's an old idea of mine and I've always liked it.
I have some other cards coming with another sub-theme but I think this is a good start for people to look over.