Hmmm. If we considered an ongoing problem in Dominion being that fast no-combo decks (BM + Smithy or Smithy variant, Doublejack, BM + Masq) are too good, I could see a Conquest theme allowing slower play. A card like:
Raid
Action
All opponents reveal their hands. Gain coin equal to the cost of the highest-value Victory card that is revealed.
(Or whatevs, not trying to balance).
Or other stuff that gives you an advantage based on your opponent having high-value green cards. So it would encourage decks which set themselves up and knock out their end conditions very quickly, and discourage, "Oh, hey, I got $8 on turn 6, I'll get a Province... and then another on turn 10, then turn 12, then turn 15."
A catch-up mechanic. If you wanted something less swingy than what's described above, the Trade Route deal of the card getting better as more types of Victory card are bought works something like this.