Regarding Kha-Beleth. I don't want to start another argument, but you're wrong here....I won't comment on other heroes, as Ment wrote, if everyone is overpowered, then no one is.
BUT! Kha-Beleth definitely doesn't belong in that category. Try playing as him yourself. It's just not interesting to play as him, and it's probably impossible to play against him, unless he uses his wave of fire skillfully. And even if it's in the expanded version. I'll give you the main argument now. If he only appeared at the end of the game, like in the campaign, that would be cool, but he runs around the map like a regular hero, and can attack you at any moment. He needs to be weakened, but in a reasonable way, if we want him to be stronger than ordinary heroes.