OC the 4670k, if you haven't already. It is still a capable performer.
Use the savings from not doing a cpu/mobo/ram swap to get a 1080Ti, wait until RX Vega actually releases, for two reasons.
1) Vega release "may" put pricing pressure on the 1080/1080Ti cards. this combined with (we can only hope) a collapse of the mining craze to help reduce gpu prices.
2) Even if there is a "slight" bottleneck from running a 1080Ti on your 4670k, your gaming fps should still see quite a bit bigger jump than getting, say, a 1070 along with a complete platform change, for less money.
As well, ram is pretty pricey right now, so that's a thing. Also, Intel will be releasing a 6 core/12 thread mainstream cpu in the near future, iirc. So that's a thing too.
At $1500 budget, you could probably get in to a 1600x/1700 cpu/ B350 mobo, or mid range X370, and squeeze a 1080 in there, maybe a 1080Ti.
In my humble opinion, for a pc build in the near future, moar cores is better. Assuming you don't want to upgrade in like a year and a half, but get some life out of the platform.
If you want do a full upgrade in the next month or two, perhaps let us know if you have a team blue or team red preference, or don't care. Pretty sure some folks around here could toss together some pretty solid builds if we get closer to your $1500 limit, especially considering you will have some parts in your current build you can use.
Like the psu you mention, for example.