But there are very simple solution to that:
Option 1 Reduce the cooldown of WotB to 100 seconds. It wouldn’t be the first time they have adjusted base skills so it works with a set (that is why at one patch they significantly increased the damage of Dust Devils). Also it wouldn’t disturb other builds too much because most of them have perma WotB anyway.
Option 2 Increase the cooldown reduction to WotB you gain from the Boon of Bul-Kathos passive from 30 seconds to 50 seconds or whatever. You said you don’t to change active skills, but in your proposal you have suggestions for changing passives, so you must be okay with that.
Option 3 A mix of the previous two.
How so?
The reason why the WW build requires density in not because of the skill itself, but because of the way Pain Enhancer works.
Hell, the whole build revolving around and being reliant on Pain Enhancer is even the reason why the build is struggling with rift guardians that do not have minions or RG’s in general… because there is no density that allows the build to benefit from Pain Enhancers attack speed bonus. It has nothing to do with WW being the only skill that benefits from the set (although I would like to see more skills being buffed by the set as well).
Pain Enhancer is the problem here and nothing else.
Giving Pain Enhancer a limit on how many stacks it can give you (so in other words: nerfing it and thereby making either less desirable or even undesirable for WW Barbs) while at the same time increasing the damage bonus to Whirlwind on the WotW set or whatever, the build would even work on low and medium density and even more, it wouldn’t have problems against rift guardians since it wouldn’t had to rely on density anymore.