I'm also thinking that the future Rider may have the key to letting Eiji transform into OOO, probably by bringing Core Medals from the future, because they recreated Core Medals in the future, like ooo5 said. I'm thinking that the future Rider could be Eiji's descendant too, and that he's another OOO, but with a new design and probably a different belt style and perhaps a different belt voice, to seperate him from the normal OOO.
But wait, how would the future Rider, and perhaps Poseidon too, have the ability to travel back in time? I guess there may be an explanation. It perhaps has something to do with their powers. They probably would use a time portal or something, created from the power of the Core Medals, like how that vortex was created in the end of the OOO TV series. Or maybe...