The potential of earthlings is insignificant.
Goku and Vegeta peaked at Buu Saga-levels before they achieved God forms which requires either a specific ritual requiring good saiyans or training with a deity like Whis.
Cell can't achieve Ultra Instinct or Ultra Ego because he lacks God Ki and lacks the hard training and lifetime of physical conditioning that Goku has.
So the only characters with potential relevant here are Freeza and King Cold. And Freeza's mutation is just Cold's but better.
In short Cell is a guy with Freeza's potential mixed in with Zenkai boosts and Regeneration. However Goku, Gohan and Vegeta all have statements of reaching Zenkai limits in the manga so Cell would reach a similar hard limit eventually. Leaving him with just being Freeza with regeneration. I guess some Goku techniques like Instant Transmission and maybe Kaio-Ken.
If he trained and could achieve forms like Freeza does then I guess he could achieve a Black Cell form after years and years of training like Freeza did but whether or not Kaio-Ken can stack with Black is completely unknown.
So in the end Cell pretty much just becomes Freeza with IT and Regeneration.
Hey wait a second...