Are you actually seeing the CPU hitting 100% in games? How about your GPU usage? What resolution are you playing at?
The 4790k isn't a worthwhile upgrade from the 4690k if you actually are being bottlenecked unless you can find one for insanely cheap. The Intel gen 7 is a depreciated platform and you wouldn't have any upgrade options on that socket. A 8th Gen i7 or i5 would get the most out of a 1070GTX. 8600k is fantastic. 7700k is a good choice if you can find one for a good price or a 'reduced to clear' prebuilt.
New CPU also means new motherboard and DDR4 RAM if you are moving up to current gen.