You already have it. The only CPUs that are better than what you have are the i7s on the LGA 2011v3 platform, which have 6, 8 or even 10 cores plus hyperthreading. Problem is there are virtually no games that scale across that many cores and threads, so gaming performance on those chips is at best similar to the 6700k and at worst worse due to the high core count chips being clocked considerably lower than the 6700k.
The Total War series is known for being very CPU intensive and also not being all that well optimized. Expecting 60FPS solid in massive battles with thousands of soldiers on each side may not be realistic. I'm also pretty sure Total War doesn't scale very well beyond 4 cores and 8 threads so moving to LGA 2011v3 won't provide much of a benefit. If you want a performance boost in Total War, all you can do is overclock.