Vol 98 gave us Kaido's devil fruit model, Seiryuu.
This might seems nothing, but it indirectly increases the chance of Yamato joining, even if it just a liiiil bit.
Kaido having Seiryuu increases the chance Yamato having Byakko.
Byakko is Neko Neko no Mi, which can complete the devil fruit number theory.
Why Byakko?
- Zou's drawing (Kanjuro summons)
- Yamato's white hair
- Yamato's higher resilience might also stems from Byakko's metal affinity
- Dragon vs Tiger theme
- A mythical "cat", a contrast of BB Pirates' mythical "dog"
In any case, since Tobi Roppo & All Stars have ancient zoan, Kaido and his daughter must have mythical zoan.
However, there's a chance that Yamato might be a Nue mythical fruit too, solely based on her hair color scheme that matches nlue pheasant color scheme. If this is the case, since the Nue from non-canon is a Tori Tori no Mi, Oda can make Yamato's Nue to be Neko Neko no Mi. Still a good hint to complete the devil fruit number theory.
This is why I believe Yamato introduced so late, because Neko Neko no Mi itself is a huge hint for next nakama.