>>43880125The link between Melmetal and Zacian, as well as the link to Mega Lucario at all (instead of just megas in general) is a huge stretch.
Melmetal being wiped out by the ultimate weapon though, and the ultimate weapon being tied to the darkest days, both check out though.
I think the far simpler answer though is that Wishing Pieces/Stars are just mega stones that didn't get charged by the ultimate weapon. We know that the Wishing Stars are actually meteor fragments, which ties into the ORAS plotline about mega evolution.
Which means that gen 8 likely takes place in the Gen 1-5 timeline, instead of the gen 6+7 timeline. Each timeline has Mega Evolution, or Gigantimax, but not both, based on if the Ultimate Weapon fired and ended the war before the Darkest Day began. In the end they are fundamentally the same thing. Additional power that affects specific pokemon due to super charged meteor fragments.
