I would like to see more of the Lycan's land. And no, not Agharta, but the actual Lycan Realm, I thought that Aghrata was too happy to be the home of the lycan Master, the transition between that city and "The three towers" was not clear so I would like to see it expanded.
The Castle was of course underused but it made sense why, Drac's not there yet so the Castle was in his "primitive form", I can't wait to see how chaos transforms the Castle into a maze, I'm sure MS will make a great tale there.
And I also think we all can agree on how EPIC was the first sight at the Castle... it was so... unexpected... I mean, you were fighting chupacabras and orcs one second before they are the anthitesis of epic, then you turn around and OMFG the music changes and you see a MONSTER FUCKING CASTLE larger than life and with the classic "vortex at the top"; then you run to it but you just don't come close, it's collossal in itself.
No, that's how you design the Demon castle.