In the spirit of the new Renaissance expansion, here is my dumb idea for what I'd do for the "Projects" stack and their respective tokens:
In general play: Projects can be used during any game, even in games without Renaissance cards (think of them like Shelters or Platinum/Colony; they COULD be used, but they're better with cards from Renaissance). There are, however, some Action cards/events which interact with Projects. If you use those cards, you have to use Projects.
Projects Setup:
Shuffle the Projects Pile and place them face down. Flip over the top card of the pile. That is the first available project.
During your Buy Phase, you may choose to buy the top card of the Projects Pile. Take that card and place it face-up in front of you, then flip over the next card in the Projects Pile. The project starts at level 0. You cannot upgrade a Project on the same turn that you gained it unless you have additional buys
All Projects have a cost associated to them, and can be upgraded by paying the same amount. When you upgrade a Project, place a Project token on the Project card. Upgrading a Project does cost you a Buy
Projects can be upgraded a total of 5 times. Once a Project has reached the fifth upgrade (which will be referred to as "Level 5"), it can no longer be upgraded.
All players are limited to One Project at a time. You cannot own a project and buy another one.
At any point during your turn, you may "discontinue" your Project. If you choose to do this, you set aside all of the project tokens and put the Project face-up at the bottom of the Projects Pile. You are then reimbursed a set amount of money equal to the cost of the project, times the number of project tokens and divided by two (rounded down)
For Example: Charles decides to discontinue his $2 Level 5 Project. He puts the card at the bottom of the Projects pile and returns all 5 Project tokens to the supply. He is reimbursed +5$ for doing so, but loses the benefits of the Project.
Here are a couple of the Project ideas that I had (Note: None of them have names because I couldn't think of any):
$2
For each level: +1 VP. At the start of your Clean-Up Phase, you may trash a number of cards from your hand equal to the number of Project Tokens.
$3
Level 1: At the start of your turn, choose one: +2 Cards, +2 Actions, +1 Buy, +2$$, or gain a Silver to your hand (the choices must be different)
Level 2: At the start of your turn, choose two: +2 Cards, +2 Actions, +1 Buy, +2$$, or gain a Silver to your hand (the choices must be different)
Level 3: At the start of your turn, choose three: +2 Cards, +2 Actions, +1 Buy, +2$$, or gain a Silver to your hand (the choices must be different)
Level 4: At the start of your turn, choose four: +2 Cards, +2 Actions, +1 Buy, +2$$, or gain a Silver to your hand (the choices must be different)
Level 5: At the start of your turn, choose five: +2 Cards, +2 Actions, +1 Buy, +2$$, or gain a Silver to your hand (the choices must be different)
$4
While this Project is in play, all cards cost 1$ less, but not less than 0 (this affects all players)
Level 1 & 2: All cards cost 2$ less, but not less than 0 (The owner of the project only)
Level 3 & 4: All cards cost 3$ less, but not less than 0 (The owner only)
Level 5: All cards cost 4$ less, but not less than 0 (The owner only)
While this is in play, when you buy or gain a Victory Card, remove one Project Token. If there are no Project tokens to remove, then put this card at the bottom of the Project Pile. (The owner does NOT get reimbursed for this)
6$
At the start of your turn:
Level 1 & 2: Gain a Silver. You may discard a card to put the Silver into your hand
Level 3 & 4: Gain a Gold. You may discard a card to put the Gold into your hand
Level 5: Gain a Gold to your hand (no need to discard anything)
There's also an Attack card that I came up that relates to Projects:
Sabotage
$6
Action-Attack-Duration
Remove one Project Token from each other players' Projects. +$ equal to the total taken (i.e. take one from the $2 Project and get +2$, etc). Set this card aside. While this card is in play, no Projects may be upgraded, and subsequent Sabotages do nothing. (This is an anti-spam method)
Edit: Did a bit of tweaking.