The 2nd editions of Base and Intrigue did a good job of improving those sets to keep up with the rest of the cards and make them better sets. However the other sets weren't changed even though many of them have issues.
IMO
Seaside
12 blanks in the box (could be a card)
Possible replacements:
Pearl Diver: Weak and doesn't interact with enough other cards. Usually a "why not" card. Idea better served by Secret Passage
Lookout: Being forced to trash one of your good cards is pretty bad. Called "the dud of the set" by Donald X.
Navigator: Weak and time consuming. Idea better served by Guide.
Sea Hag: Mean and does nothing for you (like Saboteur).
Explorer: Weak and not very exciting. Gain a Silver to hand is a nice vanilla effect that could be put on a better card.
Embargo: Unintentional attack (people might just buy it for the $2), could be replaced with an Event.
Treasure Map: Somewhat strange and not necessarily worth its own pile (has many fans though)
Pirate Ship: Weak and weird use of tokens. Trashes opponents Coppers which was one of the problems with Thief. Fun for beginners though.
Things to do with extra space:
Events?
More Durations
More uses of tokens and mats (or have a token free box). Mats could work well with Events.
Unexplored ideas:
Treasure-Duration
Duration with a chancellor effect (ie it always misses the reshuffle)
Bottom of the deck (in less trivial ways than Pearl Diver)
Alchemy
Cards to fix (not necessarily replace)
Transmute: Opportunity cost is way too high for its strength
Familiar: $3P cost adds way too much luck to games
Scrying Pool: Incredibly time consuming due to an attack it doesn't need
Philosopher's Stone: Far too difficult to get in the games it would be most useful
Possession: Really shouldn't be able to be Throned and can be quite annoying, but it's very unique and can affect games in interesting ways even when it doesn't get bought.
Herbalist: I think it's fine but a lot of other people think it's a dud
I think most of those could be fixed with Adventures style Tokens that could also be used in events (eg a "when gain, +2 buys" token for Transmute, or a "-$1 Cost" for familiar)
Donald X has said he's done with small expansions; perhaps this could be expanded to a big box (while Cornucopia+Guilds could be formalised) with the update pack being its own small box expansion.
Events costing $P is a unique design space that essentially where a $4 Treasure or non terminal could be represented by 1 card.
Prosperity
The set seriously underuses VP tokens and could have Landmarks and Events (expensive ones!) to put them to use. It could also fill the single blank card. They could also interact with Trade Route mat to put it to better use (and that card really shouldn't be Terminal, but I suppose that's what it is now).
Replaceable Cards
Loan: Called the "dud" by Donald X.
Talisman: Somewhat weak. Similar idea done better with Charm.
Royal Seal: Not interesting enough to justify a 10 card Slot especially now that Events and Landmarks exist
Goons: An attack that really isn't necessary (ie people buy the card not for the attack) on an already extremely powerful card that compares far too favourably to other cards. Definitely a candidate for splitting the effects in new cards (eg a new card could be a $6 attack, and another new card could have the below the line effect)
Quarry: Could be replaced by an event costing $P in the revised Alchemy (as well as the aforemetioned cards, possibly) but a good card
Counting House: This card seems to get better with every new set that's released and it's good in the strangest places. Only up here because it's often considered a dud
Hinterlands
Events would suit the "when gain" theme very well
8 Blank Cards which could all be Events.
Possible replacements
Duchess - a "why not" card at best, easily replaceable with a Delve style event involving Duchies
Cache - weak and easily replaceable with an event
Ill Gotten Gains - breaks the 3 pile rule and leads to degenerate games. Easily replaceable with an event.
Develop - Very, very situational and not even a key player in games where it's supposed to be good. Could be an event given how situational it is?
Farmland - a nice card but somewhat situational especially with the "exactly $2 more". Could be replaced with an event gaining a Duchy although that would remove the Farmland->Province functionarlity.
Oracle - An interesting card (not many unique cheap terminal drawers), but flawed in the same way as Spy with a weak but annoying attack.
Cornucopia/Guilds
These sets are already being sold together and could be merged into a big box.
Guilds has 7 Blanks and events would go very well with the Overpay mechanic. Pilgrimage shows that "differently named" works well with Events too.
Replaceable Cards
Cornucopia:
Harvest - Now that Courtier and Legionary exist, it looks very weak for a $5.
Hunting Party - Annoying IRL and extremely strong, outclassing another $5 (the only other blatant outclassing was Noble Brigand/Thief, and Thief was removed). Hunting Party+X leaves a lot of the kingdom behind. May be better as a $4 Village
Horn of Plenty - A great card, but an event may be more elegant than a Treasure costing $0
Guilds:
Masterpiece: Easily replaceable with an event
Sets that probably don't need to be updated
Dark Ages
Rebuild should be replaced as a super strong card that often makes the rest of the kingdom redundant, but not much else seems like it needs changing so far.
Adventures
Gear-BM may be too monolithic (like Rebuild) but not as sure and it's an interesting card. Caravan Guard's "play when attacked" could be far better served by something that isn't Delayed Peddler/Peddler. There's no good reason for the 6 blanks; if more promo events are made they could be pack ins.
Empires
No complaints
Base Set
I'm surprised Bureaucrat survived - really wordy for the base set and not strong enough to justify it.
Merchant seems really weak and uninteresting, especially next to Poacher, but not willing to call it dud yet.
I don't like how there's still blanks but not sure what to do (Events probably don't belong in the base set)
Intrigue
I really, really don't like Wishing Well (it's entirely about guessing, and Mystic seems to be a fixed version of it) but that's not a very common opinion. It's not quite as bad now that Secret Passage exists.
Patrol should probably have been the other way around (Scout, then +3 Cards) but it's fine.
Same deal as the Base Set with respect to blank cards
