There's recently been some more discussion on
"in games using this" effects, where a card's very presence in the game adds a global effect (e.g. Duchess).
There has also been a similar idea for
Environment cards that go into play immediately when gained (replacing the previous Environment) and add a "while in play" effect for all players.
A similar idea that I like a lot (and almost certainly isn't new) is just to have cards with global effects outside of the supply... sort of like the goal cards that are chosen at the start of numerous board games, except these may just be minor rule changes rather than just bonus VP.
Basic Mode: as part of game setup, randomly select one Location. This card belongs to nobody but affects all players. It's just a setup thing that changes some aspect of the game. As a design goal, the changes should have some impact on strategy, but not so wild as to completely destroy the balance of the official cards.
Asymmetric Mode: as part of game setup, reveal one Location card plus one more for each player. Each player simultaneously bids any number of VP which will be counted against them. In bid order from highest to lowest, each player takes one Location that only counts for themselves.
Note: asymmetric mode is supposed to allow for variable player powers without having crazy balance issues (the power of each one depends on the kingdom). There are probably better ways to implement an auction mechanic for the selection of individual Location cards.
So here is a Treasure Chest implementation of such cards. None have been playtested. More could certainly be made. This would also be a very easy concept to do as print-and-play because there are very few cards (only need one of each) and they don't go into the deck so they don't have to be proxied in sleeves or anything special. Also, of the ones below, only the Guilds card requires tokens (coin tokens). The rest can be used no matter what sets you own.
Tell me what you think! Are any of these broken as they are? Would any of these be un-fun? I think the most dangerous cards for balance are Circus (potentially big VP swing), Monastery (heavy game acceleration especially with more than 2 players, and also could make Rogue and Graverobber too amazing) and Soup Kitchen (makes opening hands far more variable).
Countryside (Base)
At the end of the game, you get +3VP.
Clarification: In basic mode, this effectively does nothing. This is so that there is always a chance of playing the regular game with no special rules when randomly choosing a Location.
Tavern (Intrigue)
When you empty a Supply pile, you choose whether the game ends or continues if at least 3 Supply piles are empty.When the game ends due to 3 or more Supply piles being empty at the end of your turn, you choose whether the game ends or continues.
Clarification: if the game ends due to the Province or Colony pile being empty, you cannot continue the game.
Harbor (Seaside)
At the start of your clean-up phase, you may put one card from your hand on top of your deck.
Archives (Alchemy)
At the start of your turn, you may put a card on the bottom of your deck. If you do, +1 Action.
Note: This bottom-decks instead of discards to keep Tunnel in check.
Emporium (Prosperity)
At the start of your Buy phase, +1 Buy. The second card you buy each turn costs $1 less (but not less than $0).
Circus (Cornucopia)
At the end of the game, +5VP if you have the most differently named cards in your deck.
Clarification: if there is a tie in basic mode, they all get the VP.
Frontier (Hinterlands)
When you gain a Victory card, +1 Card.
Clarification: you can't play cards that you draw due to buying a victory card. You
can play a Treasure card that is drawn by gaining a victory card from Horn of Plenty though. This bonus is not always a good thing as it may trigger unwanted reshuffles or draw cards dead.
Monastery (Dark Ages)
At the end of your turn, choose a
Supply Kingdom card pile with more than 3 cards.
Trash a copy of that card from the Supply. Move a copy of that card from the Supply into the Trash.
Clarification: moving the card does not count as trashing it, so you do not reap any on-trash benefits.
Soup Kitchen (Guilds)
At the end of your turn, take a Coin token if you
do not have any. did not buy any cards this turn.