There's also the possibility that Caesar isn't smart enough to make fruits out of other fruits and needs the full genome from a natural animal, or that Kaido didn't want his subordinates to suffer that process for whatever reason (especially if this links to his own dramatic past), or maybe Oda would simply skip that reasonable possibility (wouldn't be the first time that he draws a thick veil over something like that). We will have to wait, but this particular possibility is what I find more interesting about Kaido right now, to be honest. Not his current plot nor Rocks, but whatever relationship he had with Vegapunk and Momonosuke's fruit.