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, specifically for Magic.
For Magic, it's a bit tougher. They feed on the dead (mostly), improve themselves, sabotage others, and manipulate technology. They also have the preys-on-like theme. You could put them into a tribal set as a tribal hoser. Watch a single Edere Vir elf decimate an elf deck, might be a fun concept, but I don't think it does them, elf decks, or Magic any favors.
My first pass concept is that they exile creature cards from your graveyard to do cool things. That's nice, but it's not really cohesive as a theme. I guess they could support a “graveyard matters” set, but I'm not 100% into that. Maybe it's because I'm not very good at magic, either designing or playing, but I don't see a lot of simple, compelling cards that come from putting N things into your graveyard and then being playable for the N times that you've got something in your graveyard.
They could be a set of tough creatures (regeneration, hexproof, or just high toughness) that get bonuses whenever a creature they've dealt combat damage to dies. If those bonuses are +1/+1 counters, they can then spend them to sabotage other creatures. That gives each of them three abilities though and as someone who's never put a single counter on a Sengir Vampire, I'm not seeing it really working out.
Maybe some of these problems come because the Edere Vir don't have a weakness. Vampires have, like, a hundred of them, but sunlight is pretty popular. Werewolves have silver. Mummies have Abbot and Costello. Maybe if I took them down a notch, I could actually get some unexpected headroom.
Ah, I don't know. Anyway, they're still a work in progress, so if you've got any ideas, let me know.