I think most of us agree that the main themes would be Cooper's return to his old self. And possibly the redemption of Laura Palmer. But could there be more?
The original series had the threat of world domination by Earle sitting on his throne in black lodge. Something he failed at. I would be completey happy if season 3 is just about the redemption of two souls, but i think Lynch and Frost (again) have something else in store for us. Especially since we had the spoiler that
It could be that Bad Coop doesn't just want to retain his self in the real world. But that he too aims at world domination or world destruction with the help of the lodges. The card he showed, the search for "coordinates" and other things we know of him could hint at that. We will see.