The Fact that the Marine said they need to warn Kizaru (Who has 100 Ships). Means it's a Major Faction
Only Major Factions We know are RH Pirates, BB Pirates, Cross Guild, SH Grand Fleet, Elbaf Kingdom, BM Pirates & Revos Army
Cross Guild have no reason to get involved in such Big Scale War
BB already sent Two of his Titanic Captains to Infiltrate & most others are Protecting Pirate Island (He is also trying to Join WG, not Fight them)
Grand Fleet were Scattered & if They didn't come to Wano, then They wouldn't come to Egghead, and They aren't that Strong
Revos Army were shown couple Chapters before, it's Impossible for them to Travel all this Distance
Shanks also have no reason to Start a War against Marines, and Oda already used him to Stop Marineford War, no need for more Hype
BM Pirates suffered Big Losses, They need to first deal with those Holes before attempting to do something Crazy
So it's either Elbaf Kingdom or Man With Burn Scar Crew
However, since that Marine Ship wasn't shown Sinking in a Whirlpool (Unless Oda hid it on Purpose)
Then i bet it's Elbaf Kingdom
Oda is gonna use this Incident to Hype Elbaf & why They were Secretly Strongest Faction since always
Even World Government doesn't Challenge them