Simple, post your ideas on how he'll come back. I know its harder to theorize at the moment since we don't actually know yet how Final Days ends, but hey, be imaginative.
I'm now leaning toward the idea that he appears to categorically die and leave a body behind. His skin goes green and all, reminiscent to the imaginary tale and original Death of Superman in Superman #149. Much like his pre-FP counterpart though, The Eradicator will restore Superman using his abilities and technology. He shows up initially as a villain because he's antagonistic toward Superdad and Superboy; on one hand they're from an alternate Krypton and thus in his eyes abominations, and also Jon is a half breed, also an abomination. Later though it'll be shown he's been working in the meantime to revive the "true" Last Son of Krypton.
I'm thinking it will be something in line with this because I'm becoming more convinced that this might actually be marketed as a spiritual sequel to Reign of the Supermen. And with Jurgens penning it he'll likely rely heavily on this old ideas. Since, you know, he doesn't really have any new ones, heh.