Maybe Oda wants Z and S to fight opponents that have their own traits/stats maxed out
Kizaru - Speed, i guess Invisibility too ( Sanji's traits )
GB - sword slashes but with a black blade,
Maybe GB is also meant to be the strongest Admiral physically ( he's even taller than Akainu ), and so he has high endurance like Zoro
Makes sense if he's also more durable than Kizaru
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Maybe they won't fight and here i'm just making up parallels when there are none
But, GB having a black blade isn't that surprising cause Fuji has a confirmed Supreme Grade
Also, we know Base Kuzan and Kizaru aren't helpless as Kuzan can fist fight ( with ACoC? ) and Kizaru can sword fight and specialize in defense with barrier haki
Akainu is bound to have his own Base ability and Fuji having supreme grade seems to imply he's powerful with it in Base ( and powerful CoO )
Maybe GB's base fighting ability is also sword fighting, with strong CoA....................
..........and since Yamato and Sanji have ACoC, it can't be ruled out that Oda will give GB ACoC as well