So, I've pieced together my Buggy becoming a Yonkou theory. It's a lot of fun. For a while, I've struggled with how Buggy could do something significant enough to become a Yonkou. Then, it hit me: what is the greatest collection of Pirates you can access? Impel Down.
Who has already been to Impel Down, and escaped from it? Buggy and Crocodile. Who is currently imprisoned in Impel Down? Who is so dangerous, that the WG is terrified of him? Yes. Doflamingo. The man who knows the secrets of the Celestial Dragons.
This is how Buggy gets his Yonkou status: the same way Blackbeard did, by raiding Impel Down and freeing pirates to work under him. I also think Shanks told the WG/Gorosei about Buggy, but, I'm not sure exactly what Shanks said.
Vivi and Sabo will tie into this. Impel Down had a sovereign (Boa) helping a criminal (Luffy) enter the prison. Vivi will get captured and sent to Impel Down, and Sabo will go to rescue her. This opens up the return of... BON KUREI!!! Crocodile's Wing!
But, how do they get to Impel Down? Blueno and his Door powers. Notice the bracelets on the arms?
Cross Guild has infiltrated Cipher Pol itself. This is why Blueno was randomly teased in Film Red, because he used his Door Fruit to help Cross Guild enter Impel Down. The great trio of Buggy, Mihawk, and Crocodile, shall dismantle the corrupt World Government, and create a flashy, peaceful paradise, by making the Pirates of Impel Down their army. Imprisoning Buggy there was the worst mistake the Marines ever made.
The puny prison of Impel Down already failed to hold Buggy and Crocodile before, and, this time, the two former Shichibukai will raid the prison, and have the last laugh.