The Leader. Lex Luthor's brilliant mind and endless bank account would no doubt give him the rights to lead this rowdy squad. His desire for absolute power makes him fit in with the rest and can pull his own in a fight in his Robotic Suit. Superman has made him a strong and smart villain, and he has the prowess to lead and even manipulate his fellow squad.
The Wildcard. As the self entitled King of Asgard, Loki's ambition and pursuit for power makes him near unstoppable. A master of magic and skilled with his staff, his enemies (minus the Avengers) often find themselves on the losing side, and being a Asgardian God doesn't hurt either. His ability to follow would be in question with this crew, but given the thought of ultimate power, or causing trouble/pain to those he sees below him, he would surely tag along. For as long as he seemed fit.
The Evil. Unlike Angelina Jolie's "likeable" portrayal, Maleficent was one of the original Disney baddies. Evil, maniacal, smart, and ruthless, she showed no mercy to those who crossed her. With her fairy magic and cruel intentions, she could easy portray herself as the "black widow" or "damsel in distress" characters that the squad could need, as well as the powerful spell-caster that could turn the tides in any throw-down.
The Muscle. Not your typical brute, but with his powers and intelligence, there are few things that Magneto can not go/get/bend/break/ through, and few others that would beat him in a fight. Given the right motivation, his ambition knows no limits, and he's the type to not rest until he's accomplished what is needed. Not the most evil of the group, but he will certainly pull no punches when it comes to fighting those who threaten him, his fellow mutants, and his cause.
Unite all 4 villains the with the same and shared cause, these 4 would be unstoppable.
Thats my entry! Thanks!