The very fact that you are asking this question means that you are looking to upgrade and once it's on your mind, you will. Coffee Lake is nearly here and the 8700K doesn't sound to bad. If the numbers are correct when the proper reviews hit, then it might be worth looking as it should overclock very well across all 6 cores and 12 threads and provide a decent performance boost over the i5 4690K...Yes, you will have to fork out for a new motherboard etc...but if money is not the driver here then why not. The Z370 is just a glorified Z170/Z270 with a few more PCI-E lanes.. (Intel screwing us Z170/270 owners).
You do have Ryzen to consider as a more cost effective platform if you want to do more than just gaming, and at higher than 1080p resolution the Ryzen is up there with Intel. The 8700K will probably get to the 1700 in multi-thread...in single thread, it will be faster..In the end, I think both Ryzen (1700/1800) and the 8th series of Intel CPU's will be good for all around performance with the 8700K edging Gaming and the 1700/1800 edging productivity, though it will be a lot closer..If money is a issue, then probably Ryzen!
Finally, if you are only running a 1080p monitor, I cant see there being that much difference with your current setup, yes you might get a bit more out of the CPU/Mobo upgrade...but a 1080/1080 Ti would be a big difference..I went froma GTX 970 to a GTX 1080 and really felt the difference. Choices, choices...