I think I'd prefer a variation that doesn't rely on the single monster system such as used for summoning. The best I can think of would probably center heavily on ritual magic for creating a small swarm of weak minions, and the option for replacing them. A single big monster feels rather like some of the 3rd edition teamwork classes- those always looked to me like they were better on paper, because they relied on having several people who all had the class, or weapon, or feat. If you were the only one who took it, it didn't work. I get the same emotional reaction to the idea of the zombie master who has only a lone zombie.