Is it only me that thinks that the additional board for the two bigger expansions looks too big and would like to buy the characters separatly?

Uhm I think many people share your feelings but I don't think that what you hope for is going to happen. FFG has chosen a policy for its Talisman expansions and changing it at this point would do more harm than good.

FFG doesn't pay me to promote their products, but I can assure you that the Dungeon, even if it could look like a "watering down" of the game, actually gives it more freshness and variety.