I wonder where this Satan thing is going, too. In the official timeline, there was no need for Satan, since Drac was the end-all, be-all of evil. To me it seems like it might cheapen Drac's legacy, having someone around who is more powerful and evil than he is.
The only upside I could see to putting Satan in the mix is if Gabriel and Zobek kill him, Gabriel inherits hell's army for his own, and becomes invigorated with the sense of power he gets from this. Really can't see something like that happening, though.