Kevin 11

Kevin 11 was originally a human with the ability to absorb energy to use it for himself. But after a fight with Ben, he accidentally absorbed the Omnitrix's power allowing him to shape shift into the 10 aliens of the Omnitrix. But, he used his powers for too long and all the 10 aliens' DNA got mixed into his own DNA, turning him into a freak, permanently. But he has all the 10 aliens' DNA:
Fourarms's body structure and his left eyes
Diamond head's right arm
Heatblast's left arm
Wildmutt's lower arms
Upgrade's back
Stinkfly's wings
Ripjaw's antennae
Grey matter's right eye
Ghostfreak's eye tracks
XLR8's tail
But, his powers have a downfall. Because he has 10 aliens' DNA, he only has 1 tenth of every aliens' powers. But, by combining his powers together, it makes up for his loss of power. For example: He can combine Stinkfly's goo+Heatblast's fire to create an explosive.

Dr. Animo

Dr. Animo was once a research scientist doing research on animal when he was arrested for illegal experimentations on animals. When he was released from prison, he spent his life in secrecy, until he came out, with green skin, and a new plan, to take over the world. He has created a new device called the "transmodulater", with the ability to maximize any animal to its full potential and recreate any dead living creature, including a prehistoric dinosaur or mammoth.

Vilgax

Vilgax is Ben's toughest enemy by far. He seems to be immune to a lot of attacks. Some of them are, Heatblast's fireballs, Diamondhead's diamond shards, Fourarms' strength, cannonbolt's cannonball ram, and a lot more. He is a gigantic alien, being the same size as Fourarms, or possibly even bigger. He has immense strength and endurance, and he also has some machinery in his arms which allow his arms to grow to an even bigger size, giving him even more strength.

Part (2)

Hey... Ran out of known aliens... Now here are the villians...

Eye Guy

Eye Guy is an alien with a body covered in eyes, hence his name. But Ironically, he has no eyes on his face. He has a big eye on his stomach and he has a body covered in green eyes of every size, making it impossible for anybody to sneak up on him. He controls every eye and can see from different points of view. But, he can also use his eyes to shoot out beams and lasers. What's more, he can combine a few eyes together to make a bigger eye, producing an even more powerful beam.

Way Big

Like his name suggests, Way big is THE largest alien in the Omnitrix. Standing at a height of 100 feet tall, he was locked away deep inside the Omnitrix. He can crush an entire army with just a stomp of his foot. His massive body also gives him added strength and endurance. Giving him to the ability to protect himself from laser-based attacks from droids or fling an object far away into space.

Ditto

Ditto is an alien with the ability to duplicate himself to an unlimited amount. Even when he clones himself to any amount, his power is still not divided into fractions, which is a good thing because then each Ditto would still have the same amount of power as the oringinal Ditto. But he has a downslide which is that when one Ditto gets injured, all of the other Dittos will also feel the pain. So, if one Ditto dies off, all of the other Dittos will also die off.