As others have mentioned it depends on what you're wanting to go for. I purchased a 1080Ti several months ago and honestly, it's a great card but it's been pretty overpowered for most of the games I've been playing. So much so that I've had to use v-sync in some games to bring the fps down. You could go for the next wave of gpus but you may find yourself in the same position. As it currently stands, I don't think anything out there is utilizing the current power of gpus to their maximum, unless of course you're running a 4K setup and going for ultra on all the settings, but even then you'd probably want some sort of SLI setup.
Also, as others have mentioned, gpus are expensive at the moment because of the bitcoin mining issue.