I have similar rig and frankly, I plan on building a new PC instead of trying to upgrade. While CPU may still be fine in lot of cases, the GPU in my case is slowly being "done" with everything (mind you, I have 970 and not 980). Upgrading GPU is almost always the most straightforward way to upgrade anything, but also you're - just as me - stuck in DD3 in times where DDR5 is slowly approaching on the horizon.
If you really want to just upgrade and you're playing in 1920x1080 resolution then I guess upgrade'ing the GPU is the way to go. Not sure what's your budget, but something like 2060 Super should be good enough for next couple of years. And with CPU you can't really upgrade in meaningful way IMHO.