Do you reccommend using only vanilla assets when creating a mod, making it useable for the greatest number of people, or using assets from the DLCs assuming that most people using mods will have the DLCs anyway? The DLCs have some nice assets it would be nice to use.

add modules that edit your base esp.

for example so i make a base mod that uses assets from vanilla.
then I add an optional esp that mods my base esp.
to, say, utilize the new dawnguard models if its included in the load order.

As long as you tell people up front that a DLC is required, then it should not be an issue.

I think best practice would be to make a mod that does not require a DLC. However, your right in that the DLCs just have too many goodies to over look.

Can you make your mod in two versions? That might be a pain, but if the objects you want to use are limited, like a certain weapon or something, then it could be done easily.