So theory time...
I think Joyboy's will and Davy's will has been inherited multiple times since ages. And their successors have all gone against Imu, but not at the same time. Like... the sun and the moon.
God Valley's flashback shows Xebec's age, Davy's age and it's ending... He will apparently be killed by Roger, and Joyboy's age will begin. (Someday we'll get the same flashback for Roger till his death, and it will be epic.)
Think of it like the Dukes' deal on Zou, but on the offensive, and metaphorical "days" and "nights".
So Xebec took his shot. He missed. He failed to recruit Harald. He didn't have those two devil fruits. He was betrayed and had to rush his plan. His family was threatened. He died.
Then it was Roger. He missed too, but way later. He was too early. He was sick. He didn't have devil fruits either. He had a giant egg. He died.
So now we're way later and... both sun and moon are going at it really hard lol. 
Luffy is nearing Laugh Tale, has recruited whoever he wanted and even more than that (except Vivi, in a similar way to Harald). He has Joyboy's devil fruit (given to him by Roger's apprentice). He has two dreams. He's beaten or befriended all relevant Rocks and Roger crew members. He's at war with the WG.
Blackbeard is also nearing Laugh Tale. He has a crew, he has devil fruits, he has plans. He has Davy's fruit, I'm pretty sure. He has lots of plans and knowledge. He might also be at war with the WG lol.
So yeah, there will be an eclipse, and they will meet and fuck Imu up. And then they'll fight.