Your current system, although aging, is well matched. What are you looking to accomplish? Are you looking to scale up resolution? Or looking for better FPS in more demanding games?
If you could get a second 1070 cheaply, that might buy you some time. The 4790k is an older chip, but its not as far behind newer CPUs as you might think. It has the power to support a 1080 Ti at 2K-4K resolutions. If you run very low resolution the CPU might bottleneck just a little. The same would apply to SLI support.
Moving passed the 1070/1080 line up, SLI loses its value. A single 2080 would be the way to go as SLI gains are no longer what they used to be. But if you are going that route, you'd want to pair a new Touring GPU with a newer platform. And that requires a different budget.
So it depends on what you are trying to achieve. If you can pick up an inexpensive second 1070, it would give you a boost in most titles and buy you some time before a new system is needed. But if you are looking for a big step up in power that would last a few years, you need a new platform . So it comes down to your goals and budget.