Nvidia is probably a long way from the next generation of GPU's. As @Master Disaster mentioned, maybe a 2080 Ti Super, but that's just nothing more than a tweak up from what they have now. AMD is the closest to a new line up, but you might have to wait a while for maybe marginally better specs than Nvidia has right now, if at all.
Just get the 2080 Ti in a few weeks. There's nothing going to come by that'll beat that. Well, unless Nvidia wants to weird us out further by introducing a RTX Titan Ti Super xD.