Are there other times outside Blackbeard vs Ace where the narrator talked about events that would shake the world?
The Fleet really. Imo the whole Throne Wars thing is a 3 tiered war.
1. First was against 2 Yonko at a Raid
2. Second, imo, will be the fight for One Piece with BB/Shanks (i.e. the thing the SHF will take part of near Lodestar).
3. Finally, the fight with the Marines and WG as a World Wide event
This just sounds like setting the stage for the 3rd War in the same way Dressrosa seems to set up the fight for One Piece (with the fleet, assuming thats their use). Vegapunk will sail with Luffy (to Elbaf or Dragon, who knows), Kizaru will probably lose, and Egghead will be completely destroyed. Shaking the world propels Luffy to a deeper threat with the WG, but One Piece isnt really between the Marines and WG anyway, at least narratively.