Aksis? Nah. Rasputin should have been the final boss. We have seen his place, how it's been broken into countless of times and heard of his "underground" secrets. Imagine Rasputin being transfused with SIVA in the last section of the raid, right before our eyes, and that's who we had to fight! The possibilities for his attacks, looks and whatnot are endless! EDIT: Nostalgia would have also been present, the last boss being an old friend and ballet fan. Better than just a random archon priest imo