Forgot a very important part of the monomyth: the weapon that the hero defeats him with was given to him by an older, wiser man. (Woman for Cinderella). This older person usually has to die so the hero can face the main villain on their own. (Or just fuck off and let be sometimes).

Oh, and parental issues. They all have to have dead parents or trying to reconcile with their father. (Bilbo is the pseudo-father figure in this case).