Given the price tag of a 2080 Ti and how much more powerful they are than a GTX 970, if I was you, I would definitely wait. The 3000 series will either be a lot better or just a little bit better than the 2000 series (it's possible, but unlikely, that they're worse); so when they come out, you'll have a much easier decision to make:
If 3000 series is a lot better than the 2000 series > get a 3080 Ti or similar
If 3000 series is a little better than the 2000 series > get a reduced price 2080 Ti, if you can
It's worth noting that the 2080 Ti uses at least 100W more power than the GTX 970, so make sure your PSU is up to the task. That also means it pushes out 100W more heat too...