Anyone knows what happened to the dlc houses after oblivion.

My Conjecture:

Dunbarrow Cove: Discovered and raided. Or continues as a smugglers/pirate heaven.

Battlehorn Castle: Continues as a mercenary stronghold. Or is claimed by the legion for the war('s).

Frostcrag Spire: Odd it isn't visible from skyrim. Likely destroyed for some magical reason. Could be an oversight and it's now claimed by the synod or an independent group of mages. Given it's magical nature could it be "relocated" somewhere else?

Deepscorn Hollow: Likely a vampire heaven now. Or got ransacked by the KOTN(or some other Do-Goody faction).

No information about them at all, they are likely used by whatever faction took to them first, or they are abandoned. I'd suggest you make up your own story and stick with that, probably the most fun.

Frostcrag Spire is now a mini-tower, with the Draconian madstone as its stone.

Oh yeah, Deepscorn was probably raided by the Vigilants. They're into that stuff.

Let's hope that, with exception to Frostcrag Spire, they've all been lost to history. They were never historic to begin with.

Hey now. Battlehorn castle was pretty fun. Besides it was like the ONLY functioning castle outside of cities in all of Cyrodiil. Everything else was in unnecessary ruin...you would think the Legion would be using some of the forts outside of the towns lol