well, if kabuto learns the secrets of the so6p, i suppose he could cheat the death god somehow. those 4 hokages have the potential to wipe out the shinobi alliance by themselves, and seeing as two of them have already been shown as edo tenseis, i hope it doesn't happen. unless you're going to give them a meaningful ending, it's better not to revive them at all imo. it ruins their original death, eg deidara, kakuzu.