No real budget in mind. Mostly looking for effective upgrade per $
Most of your fps per $ will come from your GPU but you like you have that covered already with a 1080. I would say get a solid platform upgrade, 6 or 8 cores will benefit you in the future.That will also allow you to get some faster (quad channel if you can afford the board) memory - 32gb seems like a reasonable upgrade. Other than the only thing I can think of is a new GPU when next gen comes out and your case, peripherals etc. You may also want some more mass storage by getting a larger hard drive - 2tb maybe?