Hearthfire is definitely the next DLC. I wasn't convinced about Dawnguard and all the crossbow, vampire and Snow Prince stuff, but the Dawnguard reveal and trailer proved me wrong. I won't make the same mistake again.
As for what the DLC will be about though, the only thing I can possibly think of is the seasonal foilage from the GameJam.

I doubt it has anything to do with holidays from Daggerfall. It's just farfetched. And if the date of your savegame is in First Seed, you'd have to wait half a year to get to Hearthfire, so I really doubt it has anything to do with the month itself either. Perhaps it will be about something that happened in Hearthfire in ancient times, but not some Hearthfire-exclusive event.
My real guess is that the DLC is named after something that has taken its name from the month, like a pirate lord's ship ("Arrr, ready the Hearthfire and set sails for Dawnstar! We're gonna raid the crap out of that city") or some group of people (although the latter would be repetive after Dawnguard).
I argue the opposite. Declaring that you don't care what it is, you'll buy it no matter what is what will lead to one day a DLC where a duck quacks at you for 30 seconds and then deletes your saves.

A DLC to vampires svcks enough.
Haha!