Simply put, if Kurono can come back (and he will be back - everything points to that) then Izumi can be brought back too. I think Nishi might revive him or ask someone to revive him; whether you like it or not, the kid's smart, and he should know getting out of Gantz right now endangers his life more than staying in. He knew Izumi back when he was a leader, and they had this talk about Izumi wanting to beat the other teams, so he knows he'd be a good addition to the current team.
Gantzer : Izumi is a badass jerk and a Murdrer (Shinjuku's Massacre).But Lately he become a nice person (helping kurono , killing the vamps ...) , therefore , he has a respectful numbers of fans !! So , HE'LL be Back !!
Nightmare : you can take that to the bank !
Say it with me folks, all together now: "we don't know!" We do not have enough information to make a serious guess. End of story.
As for the one time we've actually seen part of the memory, can someone with a better memory confirm what exactly Nishi asked Gantz for? I believe he asked for the list of people who died, which could be a subset of the memory instead of all of it.