DLC also can't directly interact with one another.
Why do you say that? Do you realise that patches are also DLC? The features from recent patches are inteacting with DLC like Dawnguard. I bet that Hearthfire will also allow you to recruit followers from Dawnguard to use as Stewards in Hearthfire, not to mention being able to smith Dawnguard items in your Hearthfire home. The only reason why certain DLC looks like they dont interact, is because it reduces the dependencies and makes programming easier. However, there is no physical reason why DLC cannot interact with each other.