To show disrespect or Power to another Yonko, an opposing Yonko would ask the first Commander or RHM to join their crew.
Also Roger asking Whitebeard for Oden (who was like a brother/RH to WB).
Going forwards, I think it’s not too farfetched for Blackbeard to ask Zoro to join him. It fits his character and is something Oda may do to Parallel Luffy’s status as an Emperor. Shanks may even ask Zoro as a test or a joke.


Also Roger asking Whitebeard for Oden (who was like a brother/RH to WB).
Going forwards, I think it’s not too farfetched for Blackbeard to ask Zoro to join him. It fits his character and is something Oda may do to Parallel Luffy’s status as an Emperor. Shanks may even ask Zoro as a test or a joke.