The manga really should end soon, I think an alien invasion is the perfect setting to end this.
What I wanna is...why can't they make another season or two of Gantz? In truth, they really could and keep it pretty cannon. Kurono can be transported back into the room and they can continue from where they left off. Just so unsure why they can't, the anime didn't have a conclusive or drastic ending, just a filler arc that could easily be rectified