Kei leaves with Gantz

just watched (ending) again now

I've come round to the idea that Kei (perhaps making the 100 score mark) is transported to the same place Gantz goes to. The space ship or space station in the sky.

Here, he will face any and all aliens/people sent there via the trap gun.
He will also be permitted to talk to Gantz and Gantz' boss, if he has one/any.

Kei would possibly be set-up to become a Gantz himself.

I think actually even a 2nd series could have been made from this.