Detailed judging of all cards to come later. Really like the entries for a difficult prompt
Honourable Mention: Hunt has a lot in common with the card I chose as the winner for being something both varied and not close to any official card. The card you get is an inverted traveller which is a cool idea in itself. I think I'd like it better if it wasn't so wildly swingy in power - it's not hard to imagine it being dead in many games. Specifying card costs would help.
Honourable Mention: Yggdrasil - It should probably cost $10, and probably give fewer cards, and I did say I preferred cards like Riverboat to cards like Black Market in early posts. But up here because I'd replace Divine Wind with this in a heartbeat.
Runner up: Mutant - this card seems like it could have been the "Druid" of Menagerie. An option that makes it always worth buying, and from there you could have fun exploring the Ways. None come to mind as completely useless (e.g. Goat - you might only want to trash 1 card).
Winner: Rogue Sorceress - No matter what card it chooses, it is not quite like any official card (the closest being Scrap). You will probably want it as a trasher, and in doing so you will play with cards you may otherwise ignore. May not need the $6 card in setup (or the $3 but I like that you have choices).
Edit: Other judgements
Colosseum - The $1 discount strikes me as is less important than the access, although if the Event is Bury or Save than it can end up being quite a powerful project. So about paying for access to events. Which I'm not particularly fond of. I think this will often be a "sure why not" $2 even if the underlying events never get bought, and otherwise a buy with potential for buyers remorse
Postpone: Seems more like a trait but I guess wording could be clunky. Hits the mark in terms of being a bunch of new cards. I like that it's non terminal to make the opportunity cost lower (i.e. more spammable(, but I don't see that being as profound as $5s costing $3 from Riverboat. A good card nonetheless.
Chimera: The 3P cost is probably balanced with the card having a good chance of being a lost city +. It's also a bit irritating in terms of openings (Familiar is on my banned cards list). I'd like it more if it cost $6 or interacted with the Potion in some other way. Maybe I'm selling short the way you've matched a very diverse card with a balanced cost? Anyway, cool design, it just inherits a lot of the issues with the entire potion concept.
Way of the Capybara: Probably balanced - strong enough to be worth considering but not going to overtake the Kingdom. Some $4 cards come to mind as being new opportunities for Ways, like Throne Room. Otherwise just seems like more Way of the Mouse.
Designer: Both the effects are much better with non terminals. It's still an interesting card with Terminals, but with spammable cantrips it's too much of an auto-buy.
Catamaran: I like Town particularly for making me rethink Woodcutter. That has potential of happening again with this card, but in general the $3/$4 gap isn't that big, so I can imagine this being bought often in very much the same way as if it was one of the $3 cards (with a little forgettable bonus). Stil I like the design.
Charm School: For balance, and theming,I think it should require discarding a Treasure or maybe an Action. It can be very strong when considering the other "every turn bonus" projects (e.g. Canal). Always worth a look, my concern is it's just too often a must buy
Committee: Top often just a mispriced vanilla card, and the process to build the card doesn't seem all that critical.
Armaments: Not too many cards it can copy, and an issue is many of them give +$2 which makes the Vanilla effect often just sit there. Just doesn't seem that fun or strong.
Helping Hand: 5 Favours is likely to be too steep for this card to feel rewarding even when it's useful. I think I'd like it better if it did the effect of a $5 card.
Explorer: Some fun things to set up (imagining Turtle or Squirrel) but also seeing that it could end up being a fairly boring gainer. You'd want your deck to maintain its character, and Ways often don't do that.
Seafarers: Very cool design. I don't like that the first time you play it, you don't know that it will do, but that's an easy fix. Same with duration tracking. Would want to playtest in multiplayer to see whether the politics of what cards are available to be played with it are fun or bitter. It would suck for one player to play a bunch then remove the only non terminal at the end. Worth continuing to explore.
Magnum Opus: As you can often just play this first, the "Magnum Opus" effect happens too often. A $6 treasure playing $5 actions is already strong enough where the card could be retooled to make it the default with extra effort required to hit the mega effect (which is very strong indeed). I'm not really thinking much about what the picked cards are unfortunately.
Venue: Depends very heavily on the event - at $4 or less you get the extra turn events so this could be very powerful, but it could also be a dead project with Borrow, or insane with Maelstrom. Often also a +buy, so I can imagine it being bought a lot even with mediocre events and a good way to thoroughly explore the event. Generally works pretty well as a project. I think I'd mainly want to see a way to balance it so it isn't obviously weak in some games and broken in others. Maybe Colosseum was closer to the mark after all (but still the same reservations about buying access).