I'm rather new here and pretty late to the conversation I think, but commenting on the -1 action/buy idea, I think it can work with another thing I considered might be neat if it was ever implemented. The basic idea would be transformation of resources.
So if a card had -1 action, -1 coin to start, it could then allow transformation of actions/buys/coins into the others. Maybe it would need to let you play money like black market.
So you have 6 actions at the end of a turn and 16 coins, but only one buy? Maybe x number of actions can turn into one buy, and into whatever the - cost of the coin was to buy double province.
This could essentially make kingdoms with no engine into an engine using big money and converting the excess money into actions/buys. Would this actually beat big money? Who knows.
A second idea would be similar to overpaying but be "overplaying" So if you use two actions, a card does something extra? Or a card may be returned to hand which also costs 1action. So if you have 3 actions, you can play it twice. Once to play, once to return it, another to play it again.