You have a very capable system now. To see any kind of improvements, you’d need to build something around the i7 6700k.
Even then you’d need to be doing something really intensive to justify the cost. 3D modeling? H264 or H265 encoding? Cutting edge games?
If you do build something new, I’d go for 16gb of DDR4 ram, a 512gb SSD for the operating system, and at least the video card mentioned by Dee (more if you are looking to run modern games at higher resolutions than 1080p). You didn’t mention your power supply. It comes into play with the more expensive video cards. Seasonic and EVGA are making good psu’s these days.