1070 Ti , 1080 , or wait?

the 1070ti overclocked will run 1440p medium/high settings in most games at about 70-80 average fps.

The 1080 stock clock will do about this on it's own but when overclocked will provide more frames, or similar frames at higher graphic settings.

If budget is an issue, I would advise not waiting and buying the 20 series, but instead saving up and getting a 1080ti and overclocking it and having a fantastic experience in games for time to come.

I used to run a 1070 overclocked on my 1440p monitor and I could run games like AC: Origins with medium settings at about 60fps, Shadow of the Tomb Raider with medium/low on unimportant settings with high/max setting for important stuff like shadows and texture quality/detail at about 70fps on average with mostly ups and little downs.
PUBG would run fairly well on my 1070 as well with I believe medium details and higher render distance.

I now have a 1080ti overclocked and I can honestly say running every heavy hitting game I own on it at 1440p with max settings it performs like a champ. Of course, there are always certain settings where going max does nothing for visual quality but makes your FPS suffer so by turning those settings down to more reasonable levels gives me great FPS.

AC: Origins with max graphics runs an average of 90fps
Shadow of the Tomb Raider with max graphics runs average 90-100 fps.
Tom Clancy's Ghost Recon Wildlands with max graphics (an option or two lowered to more realistic levels) runs average 80-90fps.
Tom Clancy's The Division max graphics (again a few unimportant FPS killing options turned down) runs average 115fps

Anti Aliasing was either turned off or to low setting for these games since at 1440p you don't need much if any AA in these games (with exception to Division, I have AA at medium setting because it makes edges of cast shadows and stuff look smooth without cranking it up all the way but any lower and the jaggies are fierce. But if I turned of AA in that game my FPS would go up to like 125 most likely along with my fps lows improving a lot too.)

If you'd like any other kinds of info let us know.

P.S. You can get something like the 1080 or 1080ti but unless you're overclocking that Ryzen 1700 you will be CPU bottlenecked in games when using those cards.
Seeing as how you opted for 2400Mhz RAM you're CPU overclocking potential and your CPU current and overclock performance is and will be limited.

Faster RAM = Faster CPU when it comes to Ryzen.

Depends on your budget.

My son has a GTX 1070 which runs 2 x 2K monitors @ 144hz. It easily runs everything he wants (DOTA2, Pubg, fortnite, witcher3 etc).

IMHO, the RTX series are overpriced. If you can grab a good deal on the 10 series cards, I'd go for that. I think they will be relevant for some years to come.

What GPU do you have now?