Now that its confirmed Imu and Gorosei are strong fighters, its possible that Punk Hazard was the mini version of Egghead (both science islands), the next stop after Punk Hazard was Dressrosa, then the next stop could be Marijoa.
And Imu send his important men: Saturn and Kizaru, while Doflamingo send his right hand Vergo to Punk Hazard, and sacrificed Monet and then send Buffalo and Baby5, lost Caesar. Perhaps with the loss, Imu will have to sacrifice some of his men, and lose Vegapunk.
Meteorite still passes through in sliced form, birdcage doesn't stop it. It would be the same with any island destroying attack.