Nice idea, well done!
It reminds me a bit of some of
my cards that deal with Debt, e.g. Connoisseur, Ransom, Feldsher (though in the mean time they look a bit different) or
Gallery and some other cards I haven't shown (yet). An important difference is that with your [MB] cards, players need Treasures in their deck, which forces them to follow a different strategy, if they want to use [MB] cards. That can be a good thing or not, but for sure it makes them different.
Now a few notes to your cards:
First of all, if they altogether represent an Expansion, there are lot's of $2 cost cards in this set. If not mixed with cards from other sets (or only 50:50 with one other expansion), this would produce very often Kingdoms with a lot of low cost cards and not much to buy for $4 or $5.
Castle WallIsn't there a problem with the reveal for Copper? Couldn't you repeatedly reveal the same copy until the Copper pile is empty?
Secret LaboratoryI think you don't need "immediately" (see Scavenger)
Pirate's TreasureIs there a reason that you exclude higher cost Treasure cards like Platinum and Bank? I would include them, i.e.:
"...costing $3 or more...".
TrollI think the wording is correct, but I was first a bit confused about the timing. At least for me it would be clearer when the two events (this turn [MB], next turn +2 cards) are separated by "If you do..."
Counter CastleNot sure whether it is too easy for a $2 cost card to activate it via [MB] when you can gain a Copper into hand beforehand.
FerrymanThis looks a bit too strong. You can easily get remove your initial Estates and later Provinces with a cheap card and just by "spending" 1 or 2 Coppers.
Pilgrim RouteSome wording: "
Replay
a non-
Duration
Action card."
The 1 [MB] for +1 Action looks quite strong.
AtollThis seems to be strictly better than Island. It has the same cost with the same abilities, plus something.
Jungle WitchThis looks like a hybrid of terminal Junk Dealer, Ill-Gotten Gains and VP added to it. I personally would either remove the trashing or the VP, just to make it simpler (likely with a different cost).
Market TownWording: "If you do, +1 Buy and cards cost $1 less this turn".