As we all know Sabo and Ace are brothers of Luffy, both of them have the flame flame fruit, both of them have a very powerful adoptive father figure that’s a major threat to the World government (Dragon in a political sense and Whitebeard in a power sense), both of them are also seen as threats to the government themselves (Sabo because of his knowledge of Imu and the throne and Ace because of his lineage).

Looking at the way how Oda likes to parallel previous arc with the most recent example being Egghead, I could see Sabo might be captured by the government. If that happens, Luffy and Dragon will probably end up like Luffy and Whitebeard at Marineford, going to an all out war with the government, except this time, Luffy has significantly more and stronger allies.

When this war happens, Blackbeard would end up playing the same role as he did at Marineford, as an enemy to both of the warring factions. He’d want to get rid of Luffy, the marines and the revolutionaries for the same reason, they’re all threats to his place as King of the world. Blackbeard is an opportunist, he wants to take them all out at the same time, while they’re weakened from fighting each other.

However, unlike Marineford, all the strawhats would be there, with the Grand Fleet (possibly transported by Kuma), combining with the revolutionaries and the countries who are loyal to Luffy (Alabasta, Fishman island, Dresrosa, Zou, Wano), Bonney’s newly acquired army of Pacifistas, the scientific weapons possessed by Vegapunk, probably the giants after Elbaf wraps up, and maybe even the Red Haired Pirates, Luffy’s and Dragon’s armies would probably have at least 10 thousand warriors, a true force to be reckoned with.

And in the case Vegapunk manages to successfully broadcast the truth of the world to the entire world, a bunch of other countries might pledge allegiance to the revolutionaries and come to help them. Also, unlike Marineford, I think Luffy will be successful in saving his brother and then him, Sabo and Dragon could do a 3v1 against Imu.