Just read a post in the anti knockdown/give us back polestagger thread, the guy suggested to lower normal knockdown chance but make the chance 100% for headshots, I liked the basic idea of it as it'd encourage aiming more and less random spam.
Now, my suggestion is to make knockdown 100% on feet for 4+ weight weapons, 80% on feet for 3+ weight weapons, 65% on feet for 2,6+ weight weapons, 40% on feet for 2,1+ weight weapons, 30% for 1,6+ weight weapons, and 10% for anything lower than that, but 0% everywhere else, this'd make usage of knockdown more skillbased as you'd have to aim, and would slightly nerf great/long mauls as they could barely ever both overhead and knockdown.