I'm thinking that you can probably get several arcs out of this, too - the Forsaken will make good monsters of the week:

- here's a problem, we deal with it
- there are more of them
- major effort, we win in Season 4, now we can deal with these other problems
- oh shit, they're coming back

and it doesn't have to be the same character dealing with this; in fact I think it would be better if they didn't.

Your outline sounds better than the real thing...