Shivering Isles was a great experience as a whole, and very well made. I'm fairly certain Bethesda will probably release another DLC of that size, so my question to you is where would you like to go?
I'd love to visit at least part of Akavir, because we still don't know much about it and it seems really interesting. It could continue on from the main quest too, because after you defeat the dragon leader (keep forgetting it's name -_-) the other lesser dragons could flee to Akavir, as it was the home of the dragons before the half snake half human creatures ate them all. This could ignite an old conflict, and you ,being the last of the dragonborn, are called accross the vast seas to aid the Tiger Dragon and his tiger people in anwar against the snake people. Some dragons could even be your allies this time around. Also we don't have a huge idea of what sort of non-humanoid creatures dwell in Akavir. There could be prehistoric dinosaurs still roaming around. I'd at least wanna see some velociraptors xD
Also, if werewolves aren't in the vanilla game, a "Hircine's Hunt" expansion would be fantastic. You could become the game in one of Hircine's massive hunts and you have to survive in his realm, working with others that are on the run. This would at least differentiate it from Bloodmoon while still giving you the chance to become a werewolf
Udig?