Some characters, like the Enchantress or the Dwarf, have abilities that target very specific cards . With the addition of new adventure cards, these abilties get "weakened".
Why don't we expand those abilties to include cards that were introduced in later expansions?
For example the Enchantress power could be "expanded" to include one or two follower for each expansion that are declared immune to "kidnapping", respecting the idea that such follower should be somehow "pure" or "holy". Such followers could also be made "vulnerable" to the Minstrel.
A short review of existing characters with this issue:
Elf - Powers linked to Woods. Not easily expandable to include new boards. I'd leave it as it is.
Druid - see above. I'd leave it as it is.
Troll - Hasn't to roll in the Crags. Could it be "widened" to some other space in the Highlands board?
Thief - Can steal from the Purchase Deck upon encountering two board spacse or Market Day. Other Market Day - like adventure cards in the expansions? I think it could be left as it is because even with only the base game its power is 90% used in the Village or City, so the Thief is not really weakened by dilution.
Dwarf - Doesn't have to roll on Crags or Chasm, can evade in the Hills, has a +1 in the Cave, is immune to Maze. There's space for improvement!
Merchant - see Thief.
Leprehaun - Hasn't to roll in the Forest. I'd leave him as it is because he's broken enough.
Swashbuckler - Draws an extra card in the Fields, and has +1 in the Tavern. These two powers seems more related to "flavour" than character balancing, so there's no great need to empower him.
Enchantress - The Unicorn, Princess and Maid are immune to her kidnapping. Should be expanded to include other "pure" followers.
Minstrel - Can charm Maid or Princess. Other female followers that would fit theme-wise?