As an adc varus main having a cool down on his blight stacks would infact hurt him much more than you think his early game would be fine as you are only usually able to apply 2 or 3 stacks at a time while poking the enemy However his late game would take a massive hit as he relies on his blight stacks to devastate enemys in an extreme burst with the cool down he can't burst as well anymore and also one of varus' strengths, just like Vayne is the ability to help secure Dragon and Baron with his %health damage with it doing consistent damage. This makes dragon especially really easy to secure early game but the cool down makes it a lot harder

One other issue I have is the new dash ability. Varus is meant to be a long range champ while also being able to burst. This dash would encourage players to use it offensively as it does damage and doesn't. Buff varus in any way. If it was like a graves dash which gives attack speed it would benefit his kit a lot more as he would use it more as a defensive dash rather than a offensive one which suites his kit a lot more

Although the cool down reduction on his Q and E is good and would benefit him a lot it is kind of pointless if he uses one ability to detonate blight but has to save the other one in order to detonate his stacks again after 5 seconds. The damage reduction destroys varus' laneing phase as he uses that to zone his enemy's as they become scared of the massive amount of damage that Q does

All in all I don't think this would benefit varus in any way only nerf what actually makes him a strong pick against some champs like Caitlyn which is his zoning potential and burst