There is one more issue with 3rd Scenario if Luffy faces Shanks and Zoro fights Mihawk, It would easily established Shanks is much much stronger than Mihawks, which invalidates the WSS title of Zoro.
For this to happen, Oda has to setup Shanks as invalid candidate for WSS competition which we don't know if is a thing or not with one fighting with a sword.
I think there's one more better scenario if let's say Mihawk joins BB crew, i wouldn't be surprised if he tries recruiting strong Schibukai and if Mihawk joiins. Luffy can beat BB and Zoro Mihawk. Bt this also has a issue, why would Mihawk join BB. Oda will have to think of something good.