I was in the test realm and had just finished the final dungeon where you have to fight Ptolemos, mithraya, and Astraeus. I was doing a side quest in the Crab Empire where you have to find the pedistals and found this very interesting. The first pedistal was a refrence to bartleby and mother raven. This was the first refrence to that since one of the wizard city side quests Merle Ambrose gives you. The second pedistal was a refrence to Mithraya. It said she came from another world and told the people of Celestia to make hall for her. So I thought, she is the last boss of Celestia, and this is the only other place that mensions her. The world after Celestia could be the world Mithraya came from since she if from another world. I think that would be so cool since she is a evil sun angel looking creature. She could be like the ambasitor that left her world to take over others. After finding out of her defeat, they could be preparing another, more powerful one to take over wizard city. It could be the first fire world (Dragonspyre is death) where they built roads and streets over a lava surface. They didn't have to worry about that because they could fly around. The last boss could be a cheating boss that has fifth teen thousand health or something but cheats more then the boss for the school quest. Hope my idea is good.
It is a good idea but given that she was made way to easy to defeat given that other boss fights were much harder before her, I am not looking for such an easy world that she would have come from. The end boss fights are too easy, the ice boss immediately before her is hard to kill but fails to have any real offense. The bosses you mentioned that you just fought were pretty much the hardest in Celestia and in my opinion the most fun to play against. After you defeat them everything else feels like an unchallenging grind. The stuff before them was fairly challenging and could likely kill you, the stuff after was anything but challenging and simply seemed long and the fear of defeat was absent.