Well 1 or several of these things should/need to happen first:
1. Ideally, Moria, Weevil and Doflamingo are safer bets as former Shichibukai for recruitment. Hancock, Kuma and Law are less likely, but who knows. Imo, Kuma goes back to the Revo's if he survives and Hancock/Law (+Kid) ally with Luffy.
2. They find the remaining Level 6 escapees (assuming they arent with anyone else currently)
3. They recruit other former enemies of Roger/Blackbeard/Shanks (maybe the Yonko that Shanks defeated for his seat 6 years ago, Silver Axe, Ochoku, Shiki, etc)
4. They find or ally with some other underworld organizations
5. Unlikely, but maybe Oda canonizes some other former film villains like Z, Tesoro, or Bullet