Oda just used the flying six as a plot device to allow the non monster trio to shine. The real six fights with the beast pirates starts now.King‚ Queen‚ Jack‚ Hawkins‚ Apo. Drake was supposed to be the sixth‚ still wouldn't put it past him to change side again as foreshadowed by Zoro
Doubt he switches sides. He's with Koby and is a good Marine. His hidden motive has to do with Sword.
Anyhow, I do find it interesting and wonder how the fight with Apoo and Hawkins are supposed to end. I do agree that these might be far more fascinating battles if Oda gives Killer/Drake the screentime. Otherwise, it's possible they switch sides (Apoo/Hawkins) or maybe we get fun things like Brook vs Apoo or Usopp vs Hawkins (battle of wit/outsmarted each other) or something.
I don't expect those matchups but I do agree that those 5 villains are probably our next major fights, once Peros and Fukurokuju are done. Maybe BMP show up and things become hectic