-50.0+x% chance on hit to gain Swiftness, increasing your Attack Speed by 1% for 4 seconds. This effect stacks up to 15 times.
-Gain 1% Cooldown Reduction per stack of Swiftness.(Requires Rank 25)
-Each stack of swiftness grants 1% Movement Speed, 1% Critical Hit Damage and 1% Critical Hit Chance. (Requires Rank 50)
This would be a nice bump to the gem, allowing it to be more widely beneficial, but not too overly broken
This is way too powerful. IMO it would be the best gem for almost any build that utilizes ATS and CD. Just movement and increase the stacks to 20.
Other than that like already mentioned the idea itself is good but your suggestions need alot of tweaks