I have a theory gents. Took some time after this chapter and I came to a conclusion. The cp0 do indeed fear luffy and kaido. They fear luffy because he is getting stronger and kaido because he is kaido. But let's take a few things into consideration
1. Bigmom wondering if the secret of the onepiece is in onigashima
2. Marco stating that onigashima had a previous name
3. Kaido stating that wano has a huge purpose and that onigashima is "special".
4. The anime alluding to onigashima (skull) being alive
5. And zunesha pulling up makes me think that there is something in onigashima that is alive. And that zunesha is going to be the use the big sword in the skulls head. In mythology there indeed is an elephant that wielded a blade.
I think what the 5 elders are afraid of is kaido and luffys fight awakening something that's been in slumber