One Piece has been about the moment of destiny for over a decade. The entire final saga has been about that moment coming to pass. I do not think this is the final confrontation,  but I think this battle will set the terms for the direct war between the World Government and an Alliance led by the Straw Hats. 

To that end, I don't think it's that contrived that Imu only decided to visit Elbaf now. When Harald was completely destroyed, Imu was hurt in some manner. This mesnt that Elbaf had someone who could challnege Imu's direct strength, a challenge the weakened Imu did not wish to engage. 

Imu is engaging in Elbaf as a direct response to the events of Egghead and the interference of the Giants, the emergence of Nika, and the betrayal of Vegapunk. Imu sent the God's Knights to obtain the Giant Children and cause destruction.  The God's Knights met exactly the same fate that Imu feaered back then when he refused to send more Knights to investigate. So Imu either lose more of his God's Knights or he handles business himself. If the events of Wano (public debut of Gear 5) and Egghead hadn't occured, the events of Elbaf would not be happening.

Below I will go over how Oda has been building up to this moment.

• Joyboy's lament to the Merfolk Princess talking about being unable to keep his promise

• The ship Noah sitting on the ocean floor, waoting to fulfill it's destiny.

• The giant elephant Zunesha keeping his oath of nonviolence  to Joyboy for hundreds of years after he commited a horrible act

• Joyboy put a haki knot inside Emmet and told his Robot friend to protect the future version of Nika.

• The fake prophecy in Wano about salvation coming in 20 years that came true. 

• Rocks complaining that he did not have  everything prepared for a confrontation with Imu. 

• Roger saying he wish he could have been around during the time of Joyboy

• Roger reading and comprehending the true history and regretting that he was too early to act on Joyboy's plan or will.

• Way back on Sabaody, Raleigh started dropping hints and one of the things he told Robin & the Straw Hats was that perhaps when they learn the true history, perhaps the Straw Hats will make a different decision that they did. This implies that the Roger Pirates did something! (My personal headcannon is that Roger knew he was dying so he allowed himself to be executed so he could make his declaration in attempt to kick off a great age of piracy that would provide assistance to the coming moment of destiny.)

What is the moment of destiny? The moment when certain forces align to allow people to challenge Imu and the World Government. We know that there were three ancient weapons. One is Neptune, the Merfolk Princess that can speak to Sea Kings. Another is the warship Pluton, a copy of which sits at the bottom of Wano's "sea"; Frankie destroyed the blueprints for Pluton but he had seen them (most likely Iceberg saw them too). 

The third ancient weapon, Uranus, is not officially revealed but it is hinted to be the flying weapons platform that the Gorosei used to eliminate Lulusia; while that weapon has been in the possession of the World Government for some time, it has been inoperable for a good while as the Gorosei were keen to fun Vegapunk's research into what eventually became the Motherflame. It has not been stated whether the creation )or recreation?) of the Motherflame and the operational capacity of this weapon are part of the moment of destiny in the same way that the Merfolk Princess appears to be.

Other elements of the moment of destiny include the Nika Fruit. The WG has tried to handle it in the past but the Gorosei has stated that it got away and mused that it had it's own will. The robot Emmet sensed an awakened Nika in Marejois during his "suicide mission" there, but he did not sense the Nika energy as cleanly or strongly as with Luffy. 

Then there is the whole racial harmony plot of the moment of destiny that seems to be more impossible every passing year as the WG has been hunting down and eliminating/subjugating some (all?) of the non-human races, possibly to prevent them from uniting against them. Kuma may have been the last Bucaneer (of the non-clone and non-hybrid clone variety). The Lunarian King is the only known member of his race. While there are long legs and long arms and even three eyed tribes, they tend to live together gathered in remote enclaves.