So, once again I have additional thoughts that would have made the OP unwieldy. I'm putting them here (there may be more later).
Idea behind the contestA little bit more on my general thinking. Because they don't require any additional physical component, or interact with a pile of non-Supply cards that you'd generally want more than one card to interact with, I see Night cards as being a mechanic that could easily be added to a set, even a single card. This would not be true for a mechanic like VP tokens (which require a pile of physical tokens to be put in the box) or Horses (which require a pile of 30 cards that you want to interact with more than one card in the set. By contrast, a Reaction could easily be added to a set that didn't previously have one (this was done in Seaside 2E with Pirate).
Another mechanic that could easily be adopted to other sets is, as illustrated by last week's contest, Shelters. That actually didn't occur to me when I first came up with this, but I am super thrilled that the idea I wanted to do anyway turned out to be something of a continuation of a theme. Another obvious example is the Overpay mechanic. There was a previous Overpay contest in Week 131, and I prefer Night cards to Overpay cards.
I know some folks feel that adding the Night phase destroys the very clean ABC (Action Buy Clean-up) turn order, and therefore think of it as a more significantly game-altering mechanic. It's never bothered me that much (I think it's easy to remember ABC -- but sometimes there's an N).
How to make a Night card "fit" another setIdeally, your card should fit the themes of the set both in terms of flavor (e.g. Hinterlands is about travel, Menagerie is about animals), and mechanics. To me, the most obvious way to do the latter is to use a set-specific mechanic, including:
- From...
- Alchemy: Potion costs
- Prosperity: VP tokens
- Dark Ages: Ruins, Spoils
- Guilds: Coffers
- Adventures: Events, Reserve cards, Travellers, player-specific pile & penalty tokens
- Empires: Events, VP tokens, split piles, Gathering cards, Debt, Landmarks
- Renaissance: Coffers, Villagers, Projects, Artifacts
- Menagerie: Events, Horses, Exile, Ways
- Allies: Rotator piles, Allies, and Liaisons
That said, this is clearly not the only way to make the mechanic fit a set. For example, there are a few ways to design a card that feels like it would be in Hinterlands. You can also use set-specific landscapes. But--quick warning--you can't just make a card a Night-Duration and say it fits with Seaside. Nocturn has as many Night-Durations as pure Night cards.
An Event, Project, Way, or Ally with the capacity to gain a Night card you designed could absolutely work. But, again, you can't use Ghost or any existing card as part of the design.
Designing Night cardsA quick reminder. Because Night cards are played after the Buy phase, things that many Action and Treasure cards do--giving vanilla bonuses of +Cards, +Actions, +Buys, or +Coins--are all but useless on a Night card (unless you're getting them next turn as part of a Night-Duration). Official Night cards do things like gain, trash, sift your deck, and attack. Other mechanics that accrue benefits on other turns (like Reserve cards, Coffers/Villagers, and gaining non-Supply cards like Horses or Spoils) offer interesting possibilities that weren't available in Nocturn.