I think it'd be interesting to have young and current Robin on teh cover - to illustrate the change and similarities of then and now. maybe with the shadow of the Buster Call behind her, and Luffy (Gear 2 or normal) and Kamie Bushin Lucchi fighting in the foreground.
Of the current titles we have, only 'Buster Call' really fits (and as it wasn't in Vol 43, I'd have to say is the most likely title, as if it weren't going to be used having the Buster Call chapter end 43 would have been apt, seeing as it was called at the end of 42).
That being said, depending on what happens 'Escape Ship' could also work, but we have another 4-5 chapters to go that will be in the vol, so let's wait and see.