This is just my opinion, but I think if it were my system, the quad core i5 is already showing it's age I think quickly. Will it still play? Yes. Quite well on many titles.
I think you are going to start to see a shift toward multi core being the norm. That i5 is equivalent to an i3 today. It seemed like there were reports of people having trouble with battlefield 1 for example with only a quad core.
I was looking at some videos on ryzen tonight actually. There are rumors that ryzen 3 next year, will be 6 core 12 threads, basically the higher end ryzen 5 now. Then ryzen 5 would supposedly be 8 core 16 threads. Ryzen 7 something like 12 core, 24 threads, and so on. Of course these are speculation.
However, you can see the market moving that direction. Intel's lineup has gone to even the i5 being 6 core parts as mainstream. So they are seeing something as well.
All of that said, your current i5 is a good cpu. No question. I don't think you are going to see a huge difference either way with whatever upgrade you choose.
However, in my thinking, your gtx 1070 is probably equal to a new 2060 performance wise. So that is serviceable imo. In my mind, I would do the cpu/motherboard upgrade. But if you do that, I would go for the best upgrade you can. So maybe i7 8700k or better. Or you could wait for the new ryzen chips to drop next year and see what those offer.
If you did the upgrade though, you could sell your old cpu/board, save the money, then when you have a little more, you could even get rid of your card, and maybe buy a 2070 or 2080.
Again, this is my opinion. You could also upgrade the GPU now, then see what the new ryzen chips look like, and what Intel brings to the table as well. So either way you have good options. I just feel like your cpu is a limiting factor at some point that you'll want to deal with.