Interesting idea, but the potential for abuse by RMT is just too high I'm afraid (whose to say the person hiring the character won't use them for gilfarming? And if they do, would the actual original player of that character be liable for damages if the character is suspended by SE?).

I'm much rather an NPC system bringing back the Path Companions from 1.0 but using them as actual fighters in battle, as there's less room for abuse with such a system.