The Osaka team antics bring an interesting question to the surface. How many gantz teams have committed "war crimes" and for how long? Up to this point I believe the aliens have only attacked in retaliation. Is this ramp up of aliens only a response to a long history of terrorist attacks by a global gantz task force? Could Gantz be an alien lieutenant hunting refugees who had escaped to earth?
Kei is not Oka and he will not come back.
Kei's story is already fleshed out completely. Compare him in chapter 1 to his death, it's amazing character growth. His death was a extremely satisfying end to phase 1. He proves that he can be important even without a suit killing vamps left and right. Kei reconciles with Tae and develops a relationship with his brother, actually caring when he dies. Kei ends his arc as a noble man proud of his life and actions compared to glum teen he was before Masaru died. He modeled himself after Masaru in the early stages of his leadership. When Masaru is revived kei dies because he channeled Masaru's compassion and now has to give it up in the form of a death. Two similar personalities agreeing with each other doesn't make for good story. The fact that there is such a big clamor for Kei only proves that death scene made a impact on readers. A weird Kei rebirth plotline (oka) will only cheapen kei's death.
But there is so much unresolved stuff!!!
Yes, Kei has girl problems and he did regain his memory right before he died but to wish for this to be all tided up is stupid. Gantz is good story because it lives in a world of grey where there is no fairy tale endings. Like the manga says this is war and not everything is fair. To revive all the characters, defeat the bad guys, have Kei choose a girl, etc. and not have consequences would ruin the manga.
The 2nd phase is named "Catastrophe" for a reason
The 100 point alien is only the tip of the iceberg. Changing rules are a response to an incoming massive invasion with several 100 pointers. Many die ,Gantz is destroyed, and gantz members around the world organize. They discuss the events that transpired and come to the conclusion they must find out about Gantz's origins. Factions arise from the group:
Alien supporters who find the battle winless and/or intrigued by an offer by the aliens to allow them to still hunt Gantz-style on the aliens' terms.(This would be Izumi-types who get off the danger and excitement.)
Revivalists who seek the Gantz origins to develop tech to defeat the aliens and restore the world's population.
Phase 3 would be a race for the secrets Gantz hides. Who are these aliens? What about the Vampires? What is Gantz's connection with Germany? What happens with a mass copy of the world? Can the aliens be truly defeated? Is there some reason for we wouldn't want them to be?