Kamen Rider OOO has already reached its end. It’s almost as if it’s a tradition to have the main character as something more than regular human being.
With the 5 Core Medals inside of Eiji’s body, he’s slowly turning into a Greeed. He’s changed so much into a Greeed that it even directly affects the Tatoba combo.
Eiji will indeed become a Greeed by the influence of the medals. Eiji can no longer revert back into a human being.
After absorbing the other Ankh, Ankh has finally regained his former body. But in turn some of his Core Medals were destroyed. Somehow, Ankh found a way to fully complete his body.

After Date left, Gotou took on the role of Birth. Date finally comes back and transforms into Birth Prototype. Before Birth was created, there was an early prototype. They are both equally strong, but Birth Prototype is only able to use the Crane Arm and the Breast Cannon.
After Maki became a Greeed by the influence of the purple Core Medals, he wishes to destroy humanity. Even though both Maki and Eiji are becoming dinosaur Greeeds, Maki is the only one that’s called “Dinosaur Greeed”. While Eiji is labeled as “Eiji Greeed”