Imo its exactly this. There are clear hints to the world starting to fight over land and resources after Vegapunk's speech. Dragon talked about it, now roughly a week has passed and we hear Mariejoa is on fire.
I dont think Oda can logically write virtually everyone fighting in one place, but I think can set up key battles and events as we clear the rest of the journey.
1. Imo, post Elbaf, the first major one will be the war for the 4th Road Poneglyph. We sort of know its happening given how Oda seems to be framing the 4 Yonko and Marines against each other and their dialogue.
If Oda was comparing anything to Marineford and it being "cute", its this one because fundamentally this is where we probably will see Blackbeard defeat Shanks and potentially where Luffy will defeat Akainu. But its more than this. Depending on who CG recruits (more former Shichibukai), Luffy's Grand Fleet Potentially showing up, and all of the significant Marine, Blackbeard and Red Hair forces because thematically this is a Pirate vs Pirate vs Marine Battle over access to One Piece, which is far larger than just 1 Yonko vs Marines which only ended in 2 Deaths/losses. Nothing else definitively ended.
2. I think the next part is more intimate, be it Laugh Tale and it exclusively a battle between Luffy and Blackbeard's crews. It just makes thematic sense as to who directly fights for the treasure here and how. But less a war though
3. The fight against the World Government will be the biggest thing Oda has done unto his series' end and he doesnt need anyone above to do it
- Luffy's entire crew/Grand Fleet
- Possibly former Roger/Whitebeard Pirate Fleets
- Big Mom and Kaido's crews run by Katakuri and Yamato probably.
- All Supernova and former Shichibukai allies (Hancock, Kuma, etc)
- All Kingdom Related Allies (Alabasta, Sky Islanders, Galley La, Ryuugu, Dressrosa, Zou, Wano, and Elbaf)
- Revolutionaries/Vegapunk
- Allied Marines (Koby/SWORD, Tashigi/Smoker, etc) and they break with Akainu's loss possibly
Vs
- Imu
- His main 13 (or at least whoever doesnt fall in Elbaf)
- Seraphim (as Adults) + Confiscated Pacifista from Egghead
- Kong (if he does anything lol)
- Cipher Pol 0/Possibly other clones like Stussy/Weevil
- Possibly other WG characters under Kong, etc we havent met yet. I.e. it wouldnt surprise me if the WG has an army/land equivalent to the Marines
- Likely countless of allied Kingdoms and fodder (CP1-9, Blades of God, etc)
And IF somehow the Marines dont actually lose before Luffy becomes PK, then it just makes this WG side overwhelming, all the while major kingdoms are even fighting each other worldwide.
Tldr: 3 separate battles, 1 for Yonko/Marines. 1 for Blackbeard. 1 for the WG.