I wouldn't advise going for an 8/9 gen yet, if for a good deal you could get a nice breathing room with a 6700k model for extra threads and overclock that seems viable to me, so far even old models like the 4770k are still rocking, and while going for an 8th/9th would give you a nice boost and likely be safe for a good couple of years, they're still... frankensteins, refreshes of the refreshes, persisting within the 14nm process, more so the 9th, that one i wouldn't ever buy....
i'd wait for the 10th gen and see if possibly this intel 10nm dilemma is solved, that would be a real architecture change (likely) in years, theres also the odds that the 8/9 gen boards wont suppor the 10th gen, so upgrading to the 8/9 gen also could mean you'll be stuck with a mobo without any room for further upgrades, so i'd wait...
yet again theres the chance that intel will once more release the 10th as yet another 14nm refresh... then in that case i guess a rock solid 8700k with 6c/12t will certainly keep you going for a good while, im skeptical about the 9700k 8c/8t...