I like that funny running joke but Gin being the 10th TT would be actually epic if we think about it. But I don't trust that Oda will come up with this great idea.
I'd say giving the biggest villain crew of the story someone who already has friendly ties with the Strawhats isn't a great idea. Gin would no doubt become an ally in the end, and we don't need villains becoming friends every time, especially not in this predictable way.
I think it is more likely that Blackbeard has a spy somewhere else who is that 10th Captain.
We've had that a lot Post-Timeskip with Vergo in the marines, Stussy in the underworld, X Drake in the Beast Pirates, Denjiro under Orochi etc, so it would fit that someone who plans ahead of time like Blackbeard does the same.
Someone in Shanks' crew would be the most epic reveal I could think of, especially when it's time for a direct confrontation between the crews.
The revolutionaries would be cool but make no sense bc then they wouldn't need Burgess to know about their base in Baltigo.
Big Mom's crew... Smoothie would be a funny reveal that she intentionally acts so useless because she's with someone else.
But my best bet who would be the perfect match-up for Sanji based on the little information we have, and who just works in the timeline is with the marines. Ryokugyu - as a counterpart for Kuzan who is most likely a spy as well.