The only battle royal I could see happening is Red hair pirates vs SHs vs Cross guild.
The issue is quite literally everyone has a connection to multiple people with their own agendas. Ive broken it down before but here:
- Luffy has ties to Shanks, Blackbeard and Akainu, for completely different reasons. He is bound to encounter all 3. Then Zoro to Mihawk. Then the entire concept of him and Imu potentially.
- Akainu plans on fighting on all fronts, and who knows what his stance will be after Kizaru. He and Luffy are pretty much destined to clash because of Ace. Ryokugyuu told Shanks its not "time yet for them to fight", which suggests the Marines intend to fight the RHP
- The RHP (rather Shanks) intends on finding Blackbeard AND going after Luffy's fleet members AND meeting Luffy. Shanks has clear ties to the Gorosei and WG.
- Blackbeard obviously intends on encountering Shanks and Luffy. Kuzan still has a lingering plot thread that may tie back to Akainu and the Marines.
- CG's current narrative thread is the 4th RP/One Piece, but Mihawk has direct ties to Shanks. They also seem to be the connective thread to all former Shichibukai (maybe).
- The WG plans on utilizing the Seraphim to combat the Yonko, this is something stated by Kaido. They are also mobilizing the God Knights for the RA. They are also using some massive weapon to destroy whole countries
- The RA obviously has ties to Luffy and the WG.
When you think of it this way, its hard to see how Oda resolves one faction without resolving at least 1 or 2 of the others. Like Kizaru alone is easy because his real narrative threads where with Vegapunk and Sentoumaru.