Most of these suggestions are pretty silly, but there are actually a few changes I would make.
In general, I can think of three different types of changes: a) nerfing a card because it's too powerful, b) boosting a card because it's too useless, and c) tweaking a card to correct an oversight or eliminate a particularly unwanted or unpleasant edge case combination. The vast, vast majority of card changes I would approve of fall into this final category. It's okay if some cards are vastly more useful than other cards: Chapel has the greatest misbalance between cost and benefit in the game and it doesn't need to be nerfed, while something like Counting House is normally a horrible buy but it doesn't need a buff: it still gets played. For a card to really deserve a nerf or a buff it needs to be so well and truly useless such that it makes Chancellor look like a power card, or it needs to not just be must-buy powerful, but must-buy powerful in a particularly obnoxious manner. And we've established that true "must-buys" hardly exist in Dominion.
Anyway, the edge case changes I'd make, followed by a brief discussion of what cards might actually be so imbalanced as to deserve a true nerf/buff:
* Throne Room ought to have "you may", putting it in line with KC and preventing the possibility of cheating IRL (if there's an action you don't want to play). Donald X has admitted as such.
* Trading Post probably should have a "you may" as well, to prevent unpleasant interactions with Golem. This might be a worthwhile addition to the other forced-trashing cards (Remake, Apprentice, Upgrade, Trade Route), but they all present some problems that TP doesn't if you make the trashing optional. Remake you'd definitely have to do both if you do any; you should have to trash a card to get the action from Apprentice (and both the action and card from Upgrade); Trade Route is the hardest to word because you'd need to *attempt* to trash to get the other benefits, but it should also still be okay to play it on an empty hand and get the buy/money.
* Possession! I don't want to nerf Possession in general, it's far weaker than its reputation- nerfing Possession is almost as unnecessary as nerfing the already-bad Pirate Ship (seriously, anyone who wants to nerf PS doesn't know what they're doing, and needs to play some 2p games with other actions that give money). HOWEVER, Possession's interactions with Ambassador and Masquerade are legitimately disgusting- you can lose a Province with no recompense, and it's not even an attack! That does bother me. Therefore, I'd propose that any cards which are "returned to Supply" or "passed" on a Possession turn instead get set aside and returned to the deck in the same manner as trashed cards. Whoever would receive a card from Ambassador/Masquerade can then still take one from the supply. I think it's fine that you can use them to give yourself a Province or other good card, I just don't think it's fine that you can remove them from someone else's deck, especially since Possession isn't an attack.
I'm sympathetic to suggestions of "Possession can't be Throned or Kinged", since a Kinged Possession is kinda obnoxious, but that change isn't nearly as necessary and I prefer a more minimalist approach.
*To prevent the Goons-KC-Masquerade pin, maybe you only get a card from Masquerade if you have a card to pass?
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Now for actual nerfs and buffs! There are a bunch of things one could do, but there are very few things one ought to do. As has been discussed to death (and I agree), Chapel shouldn't be nerfed and if that ought to stay the same then there are really very few things which are so imbalanced as to deserve this treatment. Even super-powerful cards like Hunting Party, Mountebank, and Goons are fine by this metric. Likewise, most of the weak cards don't really need to be improved, they are almost all useful at least some of the time.
* I kinda want to improve Adventurer somehow. Maybe give $1 if you hit two Coppers? Or perhaps just slap on a +Buy?
*One thing I might consider is to boost Bureaucrat and Scout by allowing them to interact with Curses: make your opponent put a Curse on deck w/Bureaucrat, or draw up those Curses w/Scout. They're not so useless that they need it to be playable, and this doesn't actually improve them much, but this tweak seems to be in the spirit of the cards at least. I considered a stronger buff for Bureaucrat, like putting the Silver in hand, but then what do you do with Explorer?
*Speaking of, Thief and Explorer are the two cards which I find to be so useless that that actually do need a buff to be playable. Explorer is probably just a blind spot in my play-style, but Thief is actually completely useless in virtually every single Kingdom. Perhaps you can put the stolen loot on top of your deck? In your hand? Give the Thief +1 Card/+1 Action? It needs something.
*On the other end of the spectrum, there is exactly one card that desperately needs a nerf: Ambassador. Ambassador is the best card in the game, basically a true must-buy in all but Gardens situations, it's cheap as all get-out, and it's attack becomes obnoxious in a way that even the curse attacks aren't. I get why it is as cheap and powerful as it is: its power is not immediately apparent to newbies, so there's a "journey of discovery" like you have with Chapel. But once you're experienced, Chapel games stay fun and Ambassador games become dreary slog-fests where everyone goes Ambassador and the winner is usually just going to be the one who doesn't have his/her two Ambassadors clash, or show up on Turn 5, or whatever. Among good players, Ambassador is just too powerful to exist in its present form. Maybe it should cost $4? Maybe your opponent doesn't get a card if you return two- so you have to choose between maximum thinning and maximum attacking? Maybe take it even further, and say that they only get a card if you return exactly one to the supply? This would prevent the midgame from getting too degenerate. Ugh, something needs to be done about this card.
I kinda want to tone down King's Court as well, but there isn't a good way to do it: making it cost $8 wouldn't actually change much.
