MY THEORY
Decade is the supreme Rider and Decade's universe is the original dimension and the other nine dimensions are alternate and possibly made by Decade himself with his own powers. Decade possibly divided his 9 rider powers to create 9 universes for the respective riders, but reason is still unknown.
So Decade is the creator of the 10 original (including himself) hesei era
riders....
This explains why Decade can use the other 9 rider powers because originally it was his!
wow that is a really deep theory.
however, i must go against it.. the power of all riders had been explained precisely throughout the series how they got it.
Kuuga: some power of ancient warriot
Agito: power given by this guy( or gal) in white
Ryuki: creation of shiro kanzaki
Faiz: creation of SmartBrain
Blade: technology of BOARD
Hibiki: the takeshi thing...? i'm not done with him yet
Kabuto: technology of ZECT / natives
Den-O: some rider pass thingy
Kiva: inherited from fangire mommy and human daddy.
however they might change the plot y'know... its a series after all...
but changing to this thing will make previous serieses meaningless..