Just finished GC and agree that the first half was by far the better part of the show. The opening to the first half was amazing, I think it's the first one I enjoyed enough to never skip.

About the ending, I was trying to think about what happened. I think a second season is almost impossible, because Mana is able to rest in peace after starting the Apocolypse. And when Shu was absorbing the virus from the world into himself, did he get people's voids because they were a part of the virus? I think I remember that voids are bits of Mana's power that was scattered on Lost Christmas, therefore were created from the same material as the virus.