Fingers away from keyboards, because the end is nigh.
Another good job well done, everyone. The rankings we see done on the forums and Discord show that most real cards that involve Duchies aren't very viable that often, namely Duke, Duchess and Annex. Count isn't so bad. So is a ‘high rank card’ involving Duchies possible? Whether it is or isn't, it's a design space that all your entries have covered nicely. Lots of similarities as has been observed, but that likely reflects the narrowness of the space.
So the entries. Sorry if I come across a bit boring…
Investment (gubump)
$5, Treasure $2 +buy, when gain gain Duchy.
This, Scientific Society and Jungle all follow the same premise, so all can empty 2 piles out of 3 quite quickly. Investment seems to be quite similar to IGG like this, with more point gain potential and easy Estate emptying in exchange for less consistency to get $5. Gaining it more casually, there are ups and downs to getting this whenever you would get a Duchy, and it's not the best choice for economy, same coin density as Copper. Overall, I can't say anything really excites me about this.
Scientific society (Asper)
$5, 2 cards 1 action when gain gain Duchy.
So you effectively make your Duchies cantrips with this. Quite clever how this probably can still cost $5, $6 may be safer, though this really can put pile emptying pressure on; even if not a focused rush it's never wrong to get this whenever you need Duchies. Possibly more so with 2 players where the last 2 Societies are labs. So more negative than positive influence, I fear.
Mirage (hypercube)
$5, event gain Duchy onto deck, if you do 2VP.
5VP for $5 is a great deal compared with 6VP for $8, so it could be more hotly contested than Provinces even with the slow down, but seeing that empty Duchies aren't an end condition but rather this strengthens Duchies to being an alt VP slog strategy, it should make for a welcome game changer. Simple, and either effective or too strong. Couldn't say without some testing.
Archduke (segura)
8 debt, cantrip gain Duchy, on gain hand onto deck in any order.
This is certainly meant to be late game only with the debt, that’s nice. In the right situation it's a cantrip gain a $5 so it really can pay off. The on-gain effect, being on a debt card so always accessible, can have its own timely uses so can add reason to put it into your deck, as well as add a little self anti-synergy. It's not bad at all, I quite like it.
Architect (Gazbag)
$4, gain a $4 or a Duchy.
It is actually quite interesting in being a gainer that changes role going late. But I suppose that's all, as you suggest.
Countess (spiralstaircase)
$4, Reaction, +Action +Buy, when would gain a card may discard this to instead choose one of Count bonuses.
There is something going for making a gain into a Duchy instead, and converting extra buys into other things. However, it can be a Chapel on first shuffle, that later provides economy, then later Duchies, without any of Count’s setbacks, just a need for a second copy or other +buys. It feels very strong indeed.
Cultivation (Commodore Chuckles)
Trash card from hand, gain up to a $6 that shares type. In games using this, Duchies are also Actions that can be played for +2 Actions.
Duchies make for engine pieces, and this can help assemble an engine, the estates into Necropoli and Coppers into Gold, leaving you to buy the Actions. Unlike Altar the estates can't turn into good pieces, but they retain a VP purpose. It’s possibly on the strong side, but a fun package.
High council (4est)
$5, Choose: reveal Duchy from hand for +4 Cards; discard Duchy for +Buy +$5; or gain Duchy.
It makes an early Duchy exciting, great, but can get it itself. So overall it looks rather strong, giving both big draw and big $ and buy together as well.
Marriage game (Faust)
Project, $8, at start of buy phase may discard Duchy to gain Duchy, when buy gain Duchy.
Firstly, I would definitely rethink the theme! Morals, and also how is it a project? That aside, instead of getting a Province once you set up better Duchy dancing for later. So there is some good thought involved in when to get this, unlike citadel it isn't always best as the first $8 I wouldn't think. It feels like a good design.
Birthright (d)
$7, event, trash Duchy from hand, set aside prize that Duchies Inherit.
Reusing prizes isn't a bad idea. Each player gets a unique effect on their Duchies; this can feel fun but in practice probably isn't, if one Prize stands out as strong in the game (like Princess when there are no other +buys) then everyone could rush to get Birthright getting early Duchies and it can be a bitter taste for the losers. Some of the prizes are designed to stay just at one copy, like Followers, though you do get few of them. And there are 5 prizes, so not ideal for 6 player games.
Astrologer (libraryadventurer)
Night duration, $4, reveal card from discard and onto deck, if Victory +1 VP +1 Coffers. Next turn may discard card for Silver or 2 cards for Duchy.
Gets away with infinite VP potential for also giving Coffers and Silvers. There will nearly always be something in the discard, and overall makes for a payload card that first gets silvers then can go to Duchies with small hands and Coffers. Top-decked greens are discarded next turn. Very different, lots of little uses yet makes overall sense. It's good, but lacks excitement somehow.
Jungle (kru5h)
Victory, $6, 2VP, when gain gain Duchy onto deck.
This seems to be less about pile emptying than the first two, it's entirely about greening. It can't be rushed so easily. And it's rather hard to not compare it to Mirage; the latter has no added speed pressure, but this could be better balanced, which is hard to call. It comes down to personal preference here...
Proprietor (MattLee)
Reaction, $2, discard a card, everyone draws to 6. When another player gains game ending card, may discard this for Duchy.
The on-play bit feels quite nice, you can use it for draw situationally and plays after the first won't usually give a card away. Still, the Reaction is the more relevant part to this contest, and it feels sort of tacked on; it's such a narrow window for it to work in, what if it's not in hand then? So this might be nicer as a non-terminal for this reason, you can increase your odds by safely collecting more.
Blight (NoMoreFun)
Attack, $4, +2 cards each other player with 5+ cards returns one to Supply, gains one costing at most $2 less onto deck. If don't gain a card gain Curse onto deck. Replace starting victories with Duchies.
This is an innovative way to use Duchies! Though with the Duchy pile being all but empty most of the time, this will not only induce quite significant point swings but also lengthen game time. It all feels quite a lot for $4 with 2 Cards, a must buy pretty much, which is going to get rather oppressive.
Bailiff (King Leon)
$5, +2 cards +buy, reveal hand +$2 per Duchy.
Often this reveal hand for bonus format can get too strong with repeated plays. But Duchies are a convincing target, it becomes an entire deck strategy that still needs support. Unlike High Council you need to buy up some early Duchies too, so it's probably not only rewarding in game context but also rewarding to play it right. Another good one.
Pilgrim (Fragasnap)
$4, Duration, Reveal from deck until 5 actions or treasures, put up to 2 in hand, put others back, discard rest. Next turn may reveal hand of no victories for Duchy.
This is quite a powerful draw, especially if you only have 5 other actions or treasures. Or it guarantees a Duchy next turn. Victories can be sifted through easily, so the Duchy gain is easy to take. It's quite easy to make a golden deck rotating two of these. It's doing plenty and is a fairly nice late game card that's not bad early, just...something is taking excitement away? I think it's all that revealing.
Land Grant (MrHiTech)
Event, $6, +1VP gain Duchy.
A 4VP card for $6. That's not as efficient as Provinces yet more efficient than Duchies so bang on, but it doesn't add much. Mirage makes things a bit more exciting.
Conquer (Chappy7)
Event, $2 +buy, trash Duchy to gain Province.
Budget Provinces that are a bit easier to get than paying straight $7. Maybe too easy; if you only need $5s and $2s to reach Provinces you don't have to build up much.
Shortlist: Mirage, Archduke, Cultivation, Marriage Game, Astrologer, Bailiff, Pilgrim.
Runner-up: Mirage by hypercube.
Winner: Bailiff by King Leon.
The convincing reason Bailiff gives to get Duchies early adds to the game more than the other entries, I felt. A different judge would choose a different winner I'm sure, because they're all great in their own way. Congrats!