It doesn't make much sense for it to be Kuzan. Kuzan joining Blackbeard only happened at the time post-skip started. Doflamingo had heard rumours the day before Dressrosa, Burgess was questioning Blackbeard on the first day of Dressrosa over whether it was a good idea to ally with him, Sakazuki and the Gorosei only found out it had happened when Doflamingo was defeated.
Meanwhile, Gatz was shouting to the crowd at the Colosseum that Burgess was famous for being one of Blackbeard's Ten Titanic Captains. Which would imply they are all well known to the public, which Kuzan could not be.
Hmm i see, your point make sense, though, i can see that Oda just didn't introduce Kuzan as the last titanic captain for the sake of waiting for the right moment. But your argument makes sense, Kuzan being a titanic Captain should be general knowledge by now, if he was.
One of the main reasons i find Kuzan to be an unlikely candidate is due to the EOS matchups with the crew.
1 crew member on both sides are missing to make the matchups complete and Kuzan obviously won't be an enemy to the crew.
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The BB pirates have openly admitted to not trusting him, why would they make him a titanic captain? They called him an ally at best, it doesnt mean hes a crew member.
There very clearly is an unknown character who is this captain. Or, someone we do know, just not revealed to be affiliated with the BB pirates.
If i remember correctly only Burgess said that he can't bring himself to trust Kuzan, while Blackbeard was alright with it given Shiryu was a member of the Government himself.
Burgess said:
" I just can't bring myself to trust Aokiji's word on this"