I was thinking what if both worlds were actually combined, how it would change the setting? It might require a brief bit of cooperative world building, but not too much I hope.

For the purposes of this discussion we'll have to make a few rules. For the purposes of this, let's ignore season 8 Buffy monsters, as the whole things starts to get iffy once we get real winged angels and lesser gods (Elohim)

Elders are the Powers that Be. Which is near enough anyway.
Hellgods are beings that were empowered by the Hollow dozens of centuries ago.
There are two types of vampires, those possessed by demonic spirits (Buffy), and Charmed ones. both are at odds with one another. Angelus was not popular the second type and sought to actively waste the bastard, and failed.
The Source and the Underworld is a pocket dimension and sometimes tries to interfere only to find that the PTB's are a little more active, and were those beings that ran away from the Old Ones during the Primordium.
The Old Ones built the Hollows and a few other artefacts to easily empower those worthy.

What other changes or additions would be required so that we can get this going?