Who is behind it all?
And what are they planning next?
I have a feeling the Black Council is a very loose confederation. Sort of like the heirs of Kemmler; in that they seek a common goal, but that doesn't mean they like each other and won't stab each other in the back any chance they get.
I doesn't appear that every major bad guy we have seen is part of the BC. So there might be more than one party pulling strings. There could be some not so nice characters who have their own agenda and would oppose the BC for their own reasons. Harry might be forced to make alliances of convenience to fight the BC. For example, there has been some speculation in a couple of threads that at some time in the future Harry might be forced to work with a hated enemy like Nicodemus, against a more immediate threat from a BC member.
That would be fun to see, though I doubt a Harry/Nic partnership; even a temporary one, will ever occur.