I thought about some of the ones already suggested. Truthfully, they should all just stay dead.
My new, revised answer: bring back everyone who has ever died ever. If we can extend this to all life, bring them all back! ...If they get to reclaim the matter with which they were previously made, imagine all the creatures and things having nutrients and particles ripped out of their bodies. All the fossil fuels would get sucked back out...and so on.
It could be called the Great Undo.