The base set of Dominion is probably the worst (least good) set out there. Not because there are so many vanilla cards - most of them, like Village, Smithy and Market, can still be interesting because they're the bread and butter of engines. But many cards that aren't vanilla are usually too weak to bother with. Instead of 10 kingdom cards from 25, you're often just playing with 7-ish from 18-ish - you'll probably almost never buy the others.
We can do better! I would like to discuss a couple of 'mods' - fixes for the weakest Base cards so they can be competitive, so that the Base game becomes more fun to play. And while we're at it, we can also nerf the strongest cards. I'll propose some of my ideas first, and then you can all comment what could be done to make it even better! Yay!
I want to try to prevent altering costs, because altering what the card does is probably more interesting, and it's more challenging to try and balance it at the specific price point the cards already have.
DISCLAIMER: I'm not trying to say Donald did a bad job, because he still made the most awesome game in the world. I also don't want to propose that my mod (or any mod) becomes the new standard. I just want to try out a variant that's more balanced, based on today's understanding of Dominion, and see what you guys have to add to this.

So the cards that could be nerfed: Chapel, Witch, maybe Throne Room
And the cards that could be buffed: Chancellor, Adventurer, Thief, Feast, Woodcutter, Bureaucrat, Spy, maybe Moat, maybe-maybe Mine (I personally think it's fine but some others think it's weak)
Some ideas:
Chapel: Trash up to 4 cards from your hand. Each other player may gain a Silver.
So Donald has stated before that trashing 3 cards will make it too slow. Instead, you can speed up the respective engine building even further - but make it so that it helps your opponent instead. Now, every time you play the Chapel, your opponent can choose to gain a Silver. This should be enough to make the other trashing cards more competitive against Chapel; going for Moneylender and/or Remodel instead doesn't give your opponent(s) free Silvers. Flavor-wise, there's something about the church giving to the poor, or something.
Witch: +2 Cards. Each other player gains a Curse card. At the end of the game, this is worth -1VP.
The penalty gives you less incentive pursuing a Witch strategy on boards without trashing. It becomes a liability when the Curses are out, because a Moat that's also a Curse is probably not worth having in your deck. Flavor-wise, the Witch is probably cursed herself.
Throne Room: You may choose an Action card in your hand that is not a Throne Room. Play it twice.
Being unable to Throne a Throne resolves some tracking issues and slightly weakens the card. It's also more consistent with cards like Golem. It also gets the 'You may' fix it should have gotten a long time ago
Chancellor: +$2. Choose one: Put your deck into your discard pile, or trash this and each other player gains a Curse.
Optional one-shot Curser. I feel the set should have another way to give out curses, and giving that option to the weakest card in the base game seems fine. Throning a Chancellor suddenly becomes awesome. Flavor-wise... looking at his ugly face probably curses you, or something?
Adventurer: +1 Action, +1 Buy. Reveal cards from your deck until you reveal 2 Treasure cards. Put those Treasure cards in your hand and discard the other revealed cards.
Not the weakest card in base Dominion, but probably the most overpriced one in the entire game. $6 for a Moat that skips over Actions and Treasures, really? I feel it needs the Action AND the Buy to become worth it over Gold often enough.
Thief: The player to your left reveals the top 2 cards of his deck. If he revealed any Treasure cards, he trashes them. You may gain one or both of these cards. He discards the other revealed cards.
Thief is the only card that needs a buff AND a nerf - only buffing it would make it horribly overpowered in multiplayer Dominion. So now it only targets the player to your left, but it can steal 2 cards instead of 1. Should be fine, right? It's still a bit niche though, let's be honest. Any ideas? Flavor-wise: why would a Thief leave any money?
Feast: Trash this card. Gain a card costing up to $5 and a card costing up to $3.
It's a one-shot self-Develop! Being able to get two cards upon trashing one makes you not lose as much momentum. Flavor-wise: if you're in the shop at the feast, you'll probably get talked into buying something small on top of your big purchase.
Woodcutter: +2 Buys, +$2
Adding another buy gives it an edge over other +Buy cards. In strong engines, having only 1 Woodcutter and no other +Buy cards is now enough to buy lots of cool stuff at once, or even triple-Province late game. It also becomes a better Gardens enabler, giving Workshop more rivalry there.
Bureaucrat: Gain a Silver card, putting it on top of your deck. You may trash card from your hand that is not a Treasure. Each other player reveals a Victory card from his hand (or a hand without Victory cards) and puts it on top of his deck.
A bit wordy, but stronger. The wordiness works well flavor-wise. It is now a Jack of All Trades variant, that doesn't draw but attacks instead, and doesn't spy but just topdecks the Silver. It also gives the set an extra trasher; it might need one, now that there is another Curser.
Spy: +1 Action. Reveal the top 3 cards of your deck. Put one of them into your hand, discard one, and put one back. Then, reveal the top card of each other player's deck. Discard it or put it back.)
Spy now provides improved sifting, and it's still effectively a cantrip. The attack remains the same. It might involve a lot of choices - but let's be honest, it kinda always did. Should that be fixed in some way? I'm not quite sure how...
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So any thoughts about this so far? Should Mine and Moat also be fixed up a bit? If so, how?