how about hasting the cooldown timer by 10%?
I'm all for hastening cooldowns like attack speed rather than going with the awkers reduction.
That said - 40% CDR is the equivalent of 66.67% attack speed.
With % reductions you run into exponential scaling issues. 1% reduction when you were at 0% before means you're basically 1% more powerful. With damage reduction - opponents would need to do 101% of your HP in pre-mitigated damage to kill you. With 98% damage reduction they'd need to do 50 times your health in damage, and adding 1% on top of that effectivly doubles your durability from that point - requiring 100 times your health in damage up from 50, with a 1% increase.
CDR currently works on this awkward mechanic - which plays a big part in why it's capped.
If cooldown haste actually speed up casting times - placing a cap at 60% cooldown haste would be a welcome change compared to the existing 66.67% equivalent though IMO.
It would open up more diversity in league and hopefully a new CC, CDI, "Cool Down Increase" which would slow down the cooldown timer.
As intriguing as that is on a mechanics level - I fear adding such a debuff type would largely be like using mana burn or thorns. Interesting: but ultimatly not particularly great for game health.
It would be difficult to really feel lowering you're opponents cooldowns with a skill due to a heavy lack of feedback on what that's actually accomplishing. You don't get to see your opponent's cooldown timers, which would make it really easy to heavily devalue something that is perfectly balanced, causing a general sentiment it's pretty useless.
While inversely causing a massive, easy to feel impact on your opponent that's really painful to deal with. Needed you're ult for a sweet maneuver 2 seconds ago? To bad it's still on CD for 5 more seconds because you got hit with a cooldown-increasing debuff a few times 30 seconds ago.
I think Zileas refereed to this as an anti-pattern where fun fails to exceed anti-fun.
That said: lengthening casting times could be an interesting caster debuff, you'd get immediate feedback with that if you're the one applying it. It would also be substantially less irritating than Silence: because you'd still get to cast important things.