I have a feeling that I may have made this exact thread before, a long time ago. I hope not. Anyway, to continue my general habit of not posting an actual fan card, but a generic idea that could be used in a fan card....
Some of my favorite cards in MTG are alternate win condition cards. Cards that specifically state "At the beginning of your upkeep, if you _____, then you win the game." Suddenly there's a completely new goal to the game... no longer do you need to think about damaging your opponent; you just need to think about staying alive long enough to pull together whatever conditions you need. I think it would be cool if a card in Dominion did this.
Of course I realize that in some ways, alt-VP cards fulfill this niche. Heck, they even have "alternate" right there in the (unofficial) name! But still they are ultimately about having the most total VP when either Provinces, Colonies, or any 3 piles are emptied. So the game still ends in the same way. But surely there could be a fun and balanced card that creates a completely different game-end / win condition?
Just to give a very general (and bad) idea of what I mean...
Lottery - Action -
Reveal your hand. If you reveal a Gold, a Silver, a Copper, a Province, a Duchy, an Estate, and a Curse, you win the game.
Of course the problem with that example is that on almost any board that allows you to draw your deck, it becomes really easy. And on many other boards, it becomes literally impossible. The key to making such a card would be finding the balance where the condition is about as hard to reach as it is to end the game while ahead on points anyway. This way it's an interesting decision whether to go for the card or not.
Also, this may work better as an event, because there's no reason to ever buy more than 1 of those. Though the card could instead require you to reveal multiple copies of that card from your hand.
Oh yeah, and since "you win the game" isn't defined Dominion terminology (even though it's quite unambiguous), you could always get around that by having it say "you gain 1,000,000
and the remaining Provinces."