9The Flash (Earth-3)
While there is no confirmation at the moment as to who might be involved in the crossover, it seems obvious that the heroes of Earth-1 will seek aid from allies around the multiverse. Last fans heard of Jay Garrick, he had been training a replacement to protect his Earth. Despite
his willingness to retire, we'd bet Jay would be more than willing to step up to the plate when the entire multiverse is threatened. Killing Jay would allow the writers to give fans a death with little lasting impact on the status quo of any of the major series, but that would still be a shot to the heart for many fans. Given John Wesley Shipp's willingness to return to the show, it seems likely that he'll appear.