As a quick fix without the complication of changing current wpf curves, higher values in weapon master should lower the amount of wpf reduction from armor.
I was thinking that 7% less wpf reduction from armor for every WM might be good. Getting massively high WM means you can't wear enough armor for it to matter anways.
Weapon master as a skill is currently very weak. With 0 WM, you can get 110-120 wpf easily. If WM can help maintain effective WPF values, it helps buff hybrids while not buffing pure builds too much since even a small reduction hurts higher wpf values, and helps classes that relies on high WM.
This will be a nice duality between strength that helps maintain your running speed with armor, and WM that helps maintain your swing speed with armor.
Ranged:
They will be able to wear more armor without as many penalties except for running speed. They will also make more use of 60-100 wpf sidearm. They will be faster but they still won't hurt much.
Hybrids:
They will get more out of each point of wpf they have. If they have 100 wpf in another weapon, the effective wpf will be closer to 90-100 with a high WM build than it was before in the same armor.
High WM pure builds:
With 160-180 WM, they can wear more armor and keep pretty high WM. Due to their low strength, they can't wear armor that is too heavy if they don't want their movement speed to be drastically slower. Any reduction to high WPF is harsher than it is for something like 130 WPF. Swing speeds won't get any faster than a high WM build with barely any armor at all.
Low WM builds are unaffected and play exactly the same.
The average player with 5-6 WM would receive a slight-medium boost.
Possible problems:
Medium armor becomes way too good. With looms, everyone would be able to get decent armor values. This would devalue light armor a bit. Light armor would still be used mostly for movement speed anyways, though.
I believe completely reworking WPF would be better, but it will be quite complicated to get it right and still make WM viable. I just thought of this so there's bound to be problems I haven't thought about.