What if the Gorosei didn't eat a DF, but were injected with different DF abilities?
BB would be able to fully control three DFs, whereas the Gorosei would have concoctions derived from only certain DF specifics, without the entire package.
A spider zoan for locomotion, Marco's flames for regen, a minotaur's eyes for hax, etc.
Saturn already told Bonney that they'd been experimenting with the Age Age no mi. What if this isn't anything new? What if the AK had already made investigations into this field, the result of which are the current Gorosei + Imu, and the WG is trying to replicate and perfect the process. A sort of multiple green blood infusions with no time limit on its use.