I have a theory that Fuga will regain his intelligence by becoming a Homie.
One limitation of Big Mom's power is that she can't put souls in other people.
Tama has the same limitation her power doesn't work on humans or Zoans but does on SMILE users which Fuga is. I think Big Mom has the same loop hole.
Big Mom stated that she liked the Numbers and revealed that they were failed artificial ancient giants.
I think her statement will come full circle and her power will fix Fuga. Her power can personify animals and gives them human intelligence. I think this will restore Fuga's previous personality.
I think Fuga will absorb a soul by eating a Homie.
Through Hera we were shown that homies can eat another homie and absorb their soul to become stronger.
Big Mom was shown consuming her own soul to get bigger. Which I think was a hint souls can empower a human.
Big Mom did create homies while on Onigashima so I think there will be homies to eat.
It was revealed that Hatcha is 188 years ago which brings up a lot of questions. It suggests that all the Numbers are centenarians and that Fuga is 122 based on the pattern.
I think it will be related to the frozen prisoners on Punk Hazard. It was assumed that they were dead.
But I think they are still alive and was foreshadowing by this line from Impel Down about frozen prisoners being preserved for the future. I'm guessing Fuga will reveal why the Numbers were sold to Kaido and secret behind the frozen prisoners.