I do not like Bill Ferny in Edain. In MP games, most people use him as an early game fighter by leveling him straight to level 10 and only using his last ability, with which he is able to 1v1 most scouts and wreck the two first battailions. However in the books he is no more than a wicked man afraid to face Sam. That's how he should be.
I believe he could use some rework. His main features should be spying and stealing, not fighting. Here are my ideas:
~> First, remove/reduce dramatically his damage per level (after all, he doesn't level by fighting but by getting paid). Make him unable to 1v1 anyone, even Merry or Pippin.
~> Of course it would suck if he died whenever he would encounter an enemy. He would get a new ability:
Bill Ferny inspires mercy to his enemies who allow him to get off. He cannot be attacked for x seconds and gain x% bonus movement speed.
This ability would replace Snitch (closest heroes revealed). Current Snitch could be merged with Bribery, that would now spy an enemy resource building, steal a short amount of money and reveal all heroes within a certain range.
~> Last but not least, I got an idea concerning a entirely new last ability. Upon activation, Bill Ferny surrounds himself with 5 other traitors of Bree who attack who/what Bill attacks and steal resource. I like it because it would allow Bill Ferny to defend himself or even attack isolated troops by keeping close to his personality (fight only when he has the advantage).
I also suggest either to increase the amount of money necessary to level up or to put a cooldown between two pays of saboteur services, so that it is not possible to get him and level him up straight to level 10 at the start of the game.
However, if most people do not like Bill Ferny at all, I'm also fine with Mauhur and I'm sure he would be a cool scoot. It would just be a pity to lose the only reference to the spies of Bree in Isengard (if you have any other idea to integrate it in the faction, besides the scout hero?)