Thrallherd and leadership are both things that strike me as potentially bringing a lot of fun and flavor, but the person with the character needs to use it responsibly. For example, in a much more lighthearted game, I saw a person play an adolescent artificer with leadership. The fluff? His father was a well to do noble who believed in his son going out and seeing the world personally. The followers, instead of actually FOLLOWING him around everywhere, were represented as the hirelings and members of his family holdings that we could find in various towns, increasing as his level went up due to him increasing his family standing. As for the cohort? That was his warforged butler/caretaker, that he was slowly upgrading as they travelled.
But don't get the wrong idea - I'd be very, very careful about either. It's something you kinda have to know the player first before letting them choose.