This mechanic probably has some preexisting name on these forums somewhere, but I searched for "level" (the most intuitively associated word) and all I found was a bunch of talk about rankings on online Dominion games and a topic about borrowing City's interest in the number of spent supply piles.
(Since then, Poacher has also arrived on the scene.) In any case, I was thinking about Adventures earlier and its use of +1 Card, +1 Action, etc. tokens to make supply piles better, but only for individual players. Those tokens are all pretty generic because they
have to be, but what if you could tailor the tokens to
specific supply piles? So I whipped up a few quick illustrations of that idea.
Project cards are works in progress. The archetypal Project card starts out weak but can be made strong by repeatedly investing in it. When you buy a Project card, you have a choice: either you gain the card as usual, or you add one of your
Project tokens to that card's supply pile.
(This still counts as buying a card for the purposes of e.g. Merchant Guild or Contraband.) The number of Project tokens each player has on a given supply pile is referred to as that player's
level of that pile; each supply pile therefore starts out at
level 0. There is no hard limit on how many Project tokens a player can have, in total or on any given supply pile, though specific cards may stop accruing benefits after specific levels.
Redecorate is a Remodel that starts out almost useless (although the "you may" wording means it can at least kill coppers) but can be built up to an Expand or beyond.
Entourage spends a while burdening you with junk (its on-buy effect is triggered when you put a Project token on it) before letting you gain it two at a time, or better.
Boom Town gradually becomes a village or better, but also helps you with anything you may be specializing in.
Initiate is cheap but mostly useless until level 4, when it becomes a bargain Market with potential to grow even further. May be worth gaining
before leveling.
Arsonist allows you to mess with whatever cards your opponents have been leveling.
Sellsword is less effective than Militia but gains drawing abilities if other players compete with you for it.
College is a drawer that wants you to buy expensive cards before you start playing it.
Caravansary needs at least one token on it before you start, but then carries money indefinitely into the future until you finally want it.
Sunken City, like Encampment, disappears after you use it, but the moment you place a token somewhere you've got any number of sunken cities back in your deck again.
Ore is as straightforward a project card as it gets, a criminally overexpensive Copper that can be made better than Gold.
Smeltery turns coins into coin tokens, but only if you invest in it first.
Tithe gives you a lot of money in short order, but slows down the moment you get too greedy with it.
Memorial is a pricier Gold that turns into a cheaper Province.
Ramparts is a self-contained bidding war.
edit: fixed Ramparts link