Why not increase the mitigation cap from 50% to 60% with the same cost per % point. So to get to 60% damage reduction would be roughly 40k resistances. You would have to forego something in order to achieve this (no sturdy, less health, etc) and I think it would add a neat build alternative to perma blocking (ie just trying to tank with mitigation).

Would not effect PVP because at a minimum pvpers have 10k penetration so you would need 50k resistances to still have 60% mitigation. To get 50k resistance you would have to sacrafice everything including monster helm to get that much resistance and you would still die faster than the permablocker.