Ditto

Ditto does NOT make a appearnce in Ben 10 Alien force however his holagram was seen in Veggeance of Vilgax Part 1. So Ditto was unlocked in Veggance of Villgax Part 1.

GhostFreak

GohstFreak was absorbed in "Ghost Town" by Z'Skyar. But then Z'Skyar took over Ben's body.
So GhostFreak was unlocked in this episode.

Nanomech

Nanomech didn't really appear in Ben 10 Alien Force he was just made a short appearence in Ben 10 Alien Swarm(the Ben 10 Alien Force film)

DiamondHead

Diamondhead reapperis in the Veggance of villgax Part 2 after Chromostone is destroyed

Rath

Rath appears in Con of the Rath as a new alien.

Lodestar

Lodestar is a new alien in Ben 10 Alien Force He appeared in the episode Simple as a wrong transformation when he wanted to go Humagasour. Lodestar has magnet powers. He can pick up a car and throw it without touching it.

WayBig

WayBig returns in Vegance of Villgax and World of Wars.