Title asks it all... Will there be a separate CK for Dawnguard, or one single CK, or what?

Any engine changing stuff is not part of the DLC but is also patched for those who didn't buy the DLC as far as I know. So no, we won't have a separate CK I expect. The only thing DLC owners get extra is Dawnguard.esm and Dawnguard.bsa.

For example, I'm pretty sure that the Crossbow weapon type exists in the engine and CK even if you don't own Dawnguard. But Dawnguard adds the necessary Behavior files for the animations, so it's not like non-Dawnguard owners can simply model and add crossbows to their vanilla game.

It is just a case of using the existing CK with Dawnguard.esm checked off as a master, from what I understand.
Bear in mind, you then create a master dependency, but that is often done.

That makes sense... And make some editor ini changes, you're good to go. Should've thought of that meself.

Once you create the dependency, only users who have Dawnguard will be able to load your mod. So it is not something to try out without some consideration of the consequences. Just an obvious thought.