Well i won't dive in a hot debate here about how series might progress from here on... coz speculating on future plots is really boring.
But you were wrong on one thing about BB. If treated seriously by Oda, than Current BB has everything to finish off either Luffy or marines. But it's his Haki which stopping him from reaching solid high top tier level.
Also Yami fruit is already giving him a top level cheat code against all DF while Gura fruit is giving him top level destructive powers. If oda wants than with just this he can become top 3.
Seriously you made quite a good simulation here in the end. Rockss gather top level pirates at pirates paradise while BB gathered them from Impel down. Both of them also has quite a chaotic crew. Lastly Roger did beat the crap out of Rockss before finding OP so Luffy might also finishes off BB before finding OP as well.
And biggest twist is that Garp fought together with Roger to finish off Rockss and became hero while now BB has captured Coby (dubbed as new marines hero) who's also an former apprentice of Garp. So if Garp gets involves or not, but in the fall of BBPs Luffy and Coby will both be involved.
Nahh. You picked it incorrectly bro.... if not for Zoro, Luffy would have sinked MF Headquarters in the sea.
If Zoro is scared than he can stay behind.