I have two stories in mind for HoA: Demons and Elementals
Demons: In the mountains to the east of the borders of Omas, there is a research team excavating a strange set of ruins presumed to be part of Sk'liyl. At least, that's what the research team is claiming. Gray elves who have visited the site say differently. No matter who's correct, the researchers are very interested in this location. They are almost constantly transporting in mercenaries and scholars to help them take care of any minor issues, like common monsters in nearby caves, supposedly haunted locations within the ruins, etc. So far, it's been quite routine, with only one or two mercenary casualties, and those were clearly the fault of the now late warriors. Upon the arrival of the PCs, however, there will be a major discovery beyond the shore. When a team of divers returns with a set of artifacts that seem to have crashed into the sea, stranger things begin to happen.
Elementals: Some citizens of a city in the northern Ferrylands have complaints. Their crops have been failing, their favorite river is shrinking slowly, the few clouds that pass are always gray, and there is a smell in the air foul enough to make peoples' eyes water, even indoors. Compounding on these issues, the children in the area seem to be pulling more pranks than usual, and these pranks are cruel. But no child will admit to these crimes. The governor of the city is a kind, lenient man, and he suspects more than simple bad behavior. He's willing to go to great lengths, using his incredible wealth, to get to the bottom of this. After all, his own daughter has fallen under scrutiny! The investigation company he has hired has reason to suspect her, though. That's why they're beginning to call in more people. When the PCs get to the city, they will be put to work immediately doing a simple job: night watch. What they see may throw a curve ball into the case. It may also be confirmed when one of their employers is slaughtered brutally just prior to their report.
Both campaigns will be built for lvl 3-4 characters, and I expect that lvl 1-2 characters will at least be able to manage SURVIVAL. In terms of difficulty… they're both going to be HARD, but the demons arc is more likely to be CONSISTENTLY hard. Why? There will be a higher level of intelligence and tactics from the demons, whereas the elementals, while smart, won't be as coordinated. The demons will be less focused on instantaneous death, but the elementals will be more "Look! People! Kill!". All in all, it'll balance out.
And, as usual, a demonic story is going to be very dark. This is not to say that the elementals story won't have dark scenes, but demons are just dark by nature. Think… Demons are to Elementals as Batman is to the Hulk. I think that's a pretty appropriate comparison.
So, which would you rather be involved in? Any ideas/suggestions? POST 'EM! VOTE! GO! :D