More rumors have surfaced regarding OOO's possible final form, more rumors about a Reptile Greeed asides that his possible name is Iiba, and more reasons why Eiji ends up being OOO and why he is the only one who can use the Medals.
Asides from Iiba (which came from the word, ibaru which means to be prideful), another possible name for the rumored Reptile Greeed is Garu which came from the word, sugaru which means to rely or to cling on. He was born from Gil's Core Medal when they were being researched on.
The rumored final form for this series is Tajyadol Combo plus all the other Medals, and asides from that their will be a sword that OOO Tatoba Combo can exclusively use. More on Tatoba Combo, this will get an upgraded form which is like a second final form. Wait. Two final forms?!
Just like the current OOO, Ankh cooperated with the previous OOO, and they caused the God of Greeeds to scatter. Once all the Medals are gathered, they become the God of Greeeds... and the God of Greeds has chosen Eiji. Maybe this would explain why ankh insist that only Eiji can use the OOODriver.
Some minor rumor on Kamen Rider ReBirth, he is able to control over Core Medals.
Please do take rumors with a grain of salt.
Source: Netarider/Kamen Rider Rumors