For us level 100's there's not all that much we can do right now. I understand, and want, you to take your time with the new story missions to make them as cool as possible. But how about in the meantime maybe some side world?
I was thinking about maybe a new species system. To create more diversity between characters. Maybe, for instance, a new world based on Transylvania (Since Halloween and all) and it'd allow us to not only battle--but become creatures.
What if this world came with options and choices? You could battle some classic horror movie monsters and possibly join their side. Like Dracula, the WolfMan, Frankenstein and so on. Then you can decided what side to be on in a war between all these creatures: Choose Vampire and make it to Elder Choose Werewolf and make it to Alpha Choose Ghost and make it to Poltergeist So on and so forth
But like the transformations mixed in with shadow magic so they actually affect things. Maybe some new spells in the forms you take like moon magic, but instead of completely changing your deck it just adds some cards and changes some stats. It'd be fun to also walk around as a wolf or ghost whenever you so choose to.
I think I'd be a fun new experience to have a world where we can choose what side we're on, what bosses we fight, and the overall outcome of the world. We can have a more personal experience.