Grizzleheim was awesome. I loved the scenery, the characters, and the story. As well all know, aquila is based off greek mythology, having you battle the gods to test your skills. I feel there should be similar dungeons based off norse mythology. The three main gods are Thor, Loki, and Odin similar to Zeus, Poseidon, and Hades. So if this is gonna work there should be a quest to challenge them at levels 20(because that's when you're allowed back to the world.), 50, and 100(or 110). There may also be a world based of of roman mythology,(though its less popular), or maybe egyptian mythology, (as an expansion to krokotopia).
Well there are currently 2 doors in Nidavallir that aren't being used yet. I've always figured there's something big coming for them, as well as that little grendel Grumlik that stands around in the middle of the room.