Here's something that feels kind of off to me and therefore kind of Promotion-ly
Council Chamber lets you turn a card into a council room, so that you get the bonus when other people play that card. This comes at the cost of having to "lose" a copy of that card to be set aside underneath this. So, if it's a good card people want to play often, well, you're going to have to get rid of your copy of that good card.
It comes at the further "cost" of forcing you to trash a card that's in play when you yourself play a copy. This seems like a strength, but often trashing in-play cards is not ideal. Sometimes the only in-play card you will have to trash will be Council Chamber. The Council Chamber led to its own demise! This is intentional! A few rare times you'll be able to make use of this ability, like if you have ruins you want to trash or if you used some shenanigans to get coppers in play (villa, storyteller).
Without mentioning Night cards directly, Council Chamber works with them just fine. Night cards are nice because they'll let you trash coppers you have in play! Treasures work too, but you can't choose copper -- that turns out to be just too powerful. You get way too many +cards and you also turn all of your coppers into a sort of monastery. That would be over powered.
You can also set aside victory cards. That doesn't seem that useful... it's an expensive Island. However, you can set aside an estate and if other players have bought inheritances... that could be fun.
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I'm open to feedback on this. There's a lot of directions to go that are unclear. I could have limited it saying "non-treasure." I could be convinced to do that. I think saying "Action card" is less interesting, I like the ability to make this an Island or have a fun interaction with night cards.
I like the trashing part of this, but perhaps it weakens it too much. It also makes setting aside treasures under this a lot more powerful for the trashing copper ability, is that a problem?. On the other hand, I think the weakening might be important.
Also open to ways to re-word this to clarify it.
I'm now considering doing away with weakening the card with the trash in-play mechanic and then costing it at 6...