I know that you would like to keep as close to theme as possible, so I would suggest this.
Hamelin is a Tyrant, bent on causing mayhem and destruction through his Plague. By corruption of those in his proximity, he gains more followers and so furthers his own agenda. This way your still in theme, but allows you to hire other models.
Hamelin is a great master, but certain schemes and strategies are more difficult to complete with his thematic crew. Not impossible, but it is an uphill struggle.