There's a pretty large subset of the cards in the game as it exists that work as tools for solving the "too much treasure" problem. Mostly the ones that give you extra buys, but there are a few that explicitly let you set something aside for the next turn somehow.
What is it you're actually trying to fix?
I'm not explicitly trying to fix anything. I don't know whether it can be fixed within the rules of Dominion. I'm just saying that this aspect of the game introduces one of the more annoying luck factors.
Anyway, I agree that set-aside cards are great. Courtyard+BM is one of the most fun (and strongest) BM+X's. But there really aren't that many of these cards. Courtyard, Haven... and in some sense Mandarin's on-gain effect? If there were a card like CY or Haven in every set, that would be great. But in the vast majority of kingdoms, if you have too much to spend on a turn, you simply accept your disadvantage and that's that.
+buys are not a solution to the overspending problem, since $9 and 2 buys still just gets a province, $7 and 2 buys only gets two FVs if you're building FV-Wharf, etc. Also, they do nothing about the opening split, and are usually not relevant on T3/T4. It's true they do mitigate massive overspending in mid- to late-game. In my opinion, the main purpose of +buys is to make building an engine possible and worthwhile.