But realistically, at this point the October surprise can only really hurt Biden.
They could reveal trump is a rapist - it’s public knowledge, his supporters don’t care,
They could reveal he raped minors - it’s public knowledge, his supporters don’t care.
They could reveal he’s a paid up Russian/Saudi/CCP agent - it’s public knowledge, his supporters don’t care.
They could reveal he cheated on his wives while they birthed his children - it’s public knowledge, his supporters don’t care
They could reveal that Trump is a lifelong democrat - it’s public knowledge, his supporters don’t care.
There is really nothing that could come out that would move the needle on how many votes he will get.
Hell, even in the non-zero chance he dies before the election, his cult will spoil their ballots to ‘vote’ for him.