Significance of this arc
It ties into Sanji & Zoro's reaction to Luffy's near death experience at Loguetown as opposed to their optimism during the battle against the Lead Performers on Onigashima.
It allows Luffy to be "repaid" for helping and rescuing his friends who'd been in similar situations: especially Nami, Nico Robin and Nami.
Expected Plot
Fleet Admiral Sakazuki is eager to eliminate Straw Hat Luffy, whose notoriety has shamed the Marines and pushes Luffy's execution to the earliest point, possibly within the week of his capture.
With the Straw hats unable to rescue him in time, Sakazuki can flex the full power of the Marines by eliminating the recently crowned Emperor of the Sea.
Sakazuki needs this incident in order to clash with Luffy's friends and very beliefs.
Boa Hancock and Bartolomeo the cannibal are the first to get there and stall the Marines.
Leo the Tontatta and the remaining Straw Hat fleet arrive to provide Backup, forcing the marines to have to rely on Vice Admirals like Fujitora.
Soon, Alabasta, Dressrosa and Happosui kingdoms' forces team up to rescue Luffy too.
The Fishmen warrior royals also join in the fray.
Sabo and the Revolutionary Army join too. This is the most important time in this incident because it allows Sabo major character development as Luffy's brother while also pitting him against Sakazuki, perhaps Sabo fighting on his powerlessness during Ace's execution.
Koby and Sword free up Sakazuki.
The Straw Hats arrive.
The Straw Hats need to arrive because they need to flex their power and decimate the false goodwill the Marines have established: shattering the notion of justice which the One Piece world believes, falsely, equates to eliminating pirates.
This whole incident will be necessary to shift attention away from Vegapunk's revelations and prevent further info-dumping, because it deepens the stakes and allows all of Luffy's friends and good deeds to finally shine in one cathartic release in the end of the old generation of Marines.
Why now is perfect for this incident:
Blackbeard will distract the Five Elder Planets with Garp as his bargaining chip.
Shanks is making for the One Piece and might have to deal with the Cross Guild on the way.
Law and Kid had been taken down by the other two Emperors, so are MIA
Imu pressures the Five Elders to bring Vivi to her
Five Elders need to quell the mess Vegapunk's revelations made throughout the world.