I have been using a GoPro Hero 3+ with Gobe 700 dive lights for a year and half. Like the OP, I feel that I have pushed this setup about as far as it can go. The main limitations of the GoPro are its low-light performance, lack of image stabilization and fixed focal length. While it performs admirably in wide angle video in natural light, it is much more difficult to get in close and true macro is not really an option.
After looking at the Hero 5, I realized that it would not be a significant improvement over what I currently have. I further recognized that I would have to move beyond the action camera space if I wanted to surpass its limitations. Unfortunately, cost was a significant limitation for me as I do not have the means to invest $10K+ in a DSLR or mirrorless setup. Furthermore, a dedicated 4K videocamera was still not affordable at $5K+ including the housing and port.
So, I decided to compromise by klodging together an older generation dedicated videocamera and housing. I picked up a Sony HDR-580v off e-bay for $100 and a Raynox HDR-680 conversion lens for $50. I bought an Ikelite 6038.55 and 9306.1 flat port for the Raynox lens from an online retailer for $500. I also upgraded to two Big Blue 2600 lumen video lights, which I got from LeisurePro for $400 in an end of line sale. For a little over a grand, I now have a video rig with similar image quality, in-camera image stabilization and 10X optical zoom.
It should be noted that I am primarily interested in video, so this solution would not work if you wanted to shoot high quality stills. In that case, I think that an RX100 Mk2 with an Ikelite housing would offer great value for a video and still shooter.