1.Masters of Evil
The masters of evil are possibly the Avengers greatest rivals assembled
2.The Wrecking Crew
The are a desructive bunch
3.Loki
He and Hulk are the reason in the comics the Avengers assembled
4.A.I.M.
Who wouldn't want to see M.O.D.O.K.
5.Red Skull and Hydra(REBORN)
I just couldn't resist, i would love to see Red Skull twice
6.Walkers
Attack of the robots
7.Skrulls and/or Kree
Secret Invasion or Kree/Skrull war on our planet?
8.Ultron
It would probably make no sense to have Ulron as a villain and no Pyms
9.The Mandarin
Yeah he's Iron Man's villain but, think the Avengers facing off against Iron Mans deadliest foe, AWESOME
10.The Frightful Four
Law, Lyra, Trapster, Wizard, kick some ass