a Mythical Zoan is more likely because he's an Admiral . Sengoku has a Mythical Zoan and in my headcanon Kong has a Mythical Zoan too ( Sun Wukong )
but among Crocodilians , Sarcosuchus was really strong , but i'd settle for nothing less than Deinosuchus or Purussaurus
I guess chapter 1000 will be imba as we see the introduce of greenbull
there is a pattern in 4 previous Admirals introductions : they always met the Strawhats in their intros , or shortly after
• Aokiji - chapter 319 , met Strawhats there
• Kizaru - 507 , met Strawhats a few chapters after that
• Akainu - chapter 554 , spotted Luffy around 557-558 iirc
• Fujitora - 701 , met Strawhats in a restaurant
i think Ryokugyu will follow the same pattern and will appear in the end of Wano after Kaido's and Big Mom's defeats .
somewhere in 1020 - 1035 . so well within the next year