What Dominion card(s) we already know do you think have the most depth, i.e., it takes more games and thought to get the whole juice out?
I'm not sure I can give this a satisfying answer. Workshop gains a card. Deciding what to gain is like deciding what to buy; it's pretty deep relative to say the mess of choices Count gives you. The various Workshops and Remodels probably beat everything else in terms of how long you can spend getting better at them.
Aside from that unsatisfactory answer I would have to stare at the visual spoilers, and I don't want to end up saying, here are the cards I think you guys are mis-evaluating.
Currently Dominion has a really low variety of materials. Is mostly cards and in a small part some tokens and mats. Did you ever find a limitation there for an idea of a card? Did you thought about cards that used other materials but were not good enough to bother making those new materials (like a board, or counters)?
Well if you add components, you can have cards that interact with those components; it directly opens up possibilities for you. However this is better for spin-offs, because of the Alchemy issue - not slow-to-resolve cards, the other one, that some people don't like potions. Doing individual cards like Native Village and Embargo has the issue of, we can only include so many extra components for individual cards.
For the most part I have not ruled out cards due to needing components - I just haven't tried to think of cards that required components I wasn't going to get to have. I can think of one exception. I playtested "+1 handsize for the rest of the game" as an Alchemy card. It would have required a playmat - yes even if you personally wouldn't have needed one - and that killed it.
How important is to you that every card works with 2 and with 3+ players exactly as written, without referring explicitly to how many players there are? For instance, there are things, like changing the order, which don't make sense in 2 player games. Would you completely rule out a card for that? How likely would it be to have a card that cannot be used in 2 player games (or ruled out for any other number)? How about having different rules, or exceptions, to handle a specific amonut of players?
If a card wouldn't work in two player games I would not make it. It's okay for the card to refer to the number of players though, if it has to, although generally it wouldn't. Like, Tribute originally looked at the top card of each adjacent player. To work in two player games it just goes left. But at the time it put the card back on top. It could have ended up looking at the top card of each adjacent player and discarding them, and in two player games that would just mean their top two cards, with no special rules.