I'd guess a traveler line or two. Dark Ages introduced the concept of cards that upgraded to other cards, Adventures just made it its own mechanic with a keyword. If this is going to bring back Dark Ages themes, travelers make sense. And helps account for 500 cards.
I'm expecting one of the following two variations:
1. A Crown-like (Night) action/treasure, which has different effects, depending on whether it's played in the Night phase or the regular Action/Buy phase.
2. A traveler variant/madman-mercenary type thing, where you buy a regular action card from the supply, and when you discard it from play you (can/have to) replace it for a night card (not from the supply). When you play the night card, it goes back to the pile it came from, and you gain an action card again from the supply. Maybe you set aside the action card to make sure you don't suffer a net loss, instead of returning it to the supply? Something like this would work great for a werewolf type thing.
I'm really looking forward to hearing more about this mechanic.