I really really wanted Croc secret to be that he was a woman, I feel like Oda could really develop that well with how he tackled Kiku in Wano.
That is not something he tends to write, and it shows how he changed his view on that matter ever since the Okama gags from Bon Clay and Ivankov to Kiku.
However it's 100% not that, what Oda like the most is being unpredictable, he said so.
He makes a villain people see as weak stronger and so on.l to throw people off.
I just hope it's a good secret.