Wait for prices to drop, not the next gen. There's not gonna be a card that surpasses the 1080 ti performance as much as it did to the 1080 and still fall into a price underneath the titan lineup. Sure it will be better, but we have seen the launch of the best titan cards barely surpassing the 1080ti. However, if they make a gtx 1080ti equivalent with less power draw next gen, then get that if it is reasonably close to the $700-800 price range
Gtx 1080ti for 1440p 144hz is great on most games I play on ultra detail. Games like watch dogs 2 and ghost recon wild lands won't really hit above 120fps all the time, but for the most part, my other games like gta, rainbow 6 siege, pubg, destiny 2 reach my 144fps.