So a few weeks ago I started talking about a monster type I made called the Edere Vir. Originally envisioned as a part of the World of Darkness, they were humans that preyed on other humans for power and advantage within society. I'm still turning the idea over.
As a D&D monster, they wouldn't be a monster so much as a template applied to a species already found in the Monster Manual. A few regeneration abilities, a severe bite attack limited to members of their own race, and an attack or two that induces a status effect would probably be enough.
For full-on NPCs (or even PCs) the entire point is that they're better than non-Edere Vir members of their races. That means they're either races based on existing races that trade some racial abilities for a few Edere Vir abilities, they're extensions on existing races which add drawbacks and related Edere Vir abilities, or they act like an innate multiclass.
I like the latter, which requires they take a level 1 feat for abilities comparable to a multiclass feat. They can then take the adept, novice, and initiate-equivalent feats to choose from a list of Nth-level Edere Vir encounter, daily, and utility powers to replace a class power.
I don't know if 4th Edition treats add-ons or powerful races like multiclass-only classes, but it's a good system. I do kind of like the level cost system of 3rd Edition, but that might be the only thing form 3rd Edition I'm really fond of.