Additional rule: After the kingdom cards are chosen, each player (starting with the 1st player, or in secret) chooses one of the heroes. He can use abilities of his hero for the whole game. Two players can have the same hero.
That is the basic idea. There will be some hero cards from which you choose the one you find most usefull for that kingdom. Now here are some of my ideas for heroes:
Priest - Once in a turn, you may play any action card as "Trash 2 cards from your hand."
The name is reference to Chapel and Bishop.
Mayor - Once in a turn, you may discard a card. If you do, +1 action.
The name is reference to Villages.
Merchant - Once in a turn, you may discard a card. If you do, +1 buy.
The name is reference to Market.
Count - If you didn't play any card this turn, you may gain a Curse. If you did, gain a Duchy. If you didn't, you may gain 2 Estates. (so basicly you can gain Curse+Duchy, Estate+Estate, Estate (if it is last) or nothing, but you cannot gain Duchy)
The name is reference to Baron and Duke.
Judge - You can play any action card as "+1 action, +1 card"
The name is Judge, because you make a decision whether you want this card or not.
Feel free to comment the names, the general idea and the cards themselfs. Also feel free to playtest, make your own "heroes".
PS: I was thinking about "Sorcerer" that would be giving other players Curses, but I cannot think of any cool and simple idea. Thoughts? "Pirate" that steals treasures sounds cool too...