I'm thinking at least one of the heirlooms will be a Night-Treasure, because we haven't seen that dual-type yet.
It seems pointless as there isn't much of a mechanical difference between Treasures and Night cards anyway.
There are cool things you could do. The first thing right off the top of my head:
Treasure: worth $1
Night-Duration: set aside a card from your hand face down under this. If you do, at the start of your next turn put it into your hand and trash this.
So a kind of self-trashing-Copper one-shot Haven.
Save kind of already does this though. Maybe a treasure - night - duration which as treasure is worth $2, and as night gives you +1 card at the start of your next turn. But this isn't the fan card subforum.
I wonder if there will be
-A night-victory card (green-black probably looks neat though).
-Something that reduces the cost of cards.
-Interaction cards other than fool (probably yes).
-A card that says "choose X of the following:"
-A card that says "when another player plays an attack card..." (maybe not since we already have faithful hound)
-A card that costs something other than 0, 2, 3, 4, 5, 6.
-A card which specifically states duchy or province (all other basic supply cards have already been mentioned)?
-A non-treasure card that gives +$x (probably yes, but none have been previewed).
-A non-victory kingdom card with more than 10 copies
Et cetera.