8700k is the way to go if you want to build a pc, i understand the urge
Either gpu will do, perhaps maybe 1080ti, will just have to look at benchies in the games you want to play to determine that.
Still i think your current cpu is still very capable and wouldn't bottleneck any gpu you get really. Not enough to justify the cost of a new setup but if that's not an issue and you want to build a new pc go forth albeit probably not necessary with your current setup.