Actually now that you mention it. Greenbull and Fujitora kind of have a Mr.1/Mr.2, Jabra/Kaku, Queen/King dynamic don’t they?
Sanji vs Greenbull (food and women) and Zoro vs Fujitora (stoic swordsman) makes sense.
Also, I think it’s heavily implied in Dressrosa that Fujitora and Greenbull got their fruits after becoming admirals.
My headcanon is that they’re kinda to the admirals what Blackbeard is to Yonko. A notch weaker at first, but come EOS they’ll be up there.