If you reduce CDR by 20% on all items, the game would probably be a lot healthier.
If all 10% items/runes/abilities/buffs offered 8%, and 20% offered 16%, it would take 1 whole extra item to get to the cap.
Seeing how you want to put CDR ON EVERYTHING now a days to make it URF deathmatch, i think this would be the most sensible nerf to both nerf mobility via abilities, combo rotations coming up too quickly, create windows of counterplay and reduce snowballing all in one go.
The alternative would just have to be remove CDR from half the items but i dont see Riot doing this.
on Nasus and you instead have 1 - .8 x .8 attack speed for a total of 36% instead and the 6% over is halved to 33%. Throwing in a spirit visage would bring it to 1 - .64 x .9 = 42.4% with the 12.4% over being halved to 6.2% for a total of 36.4%. If someone wanted to do a build in such a system with ionian boots and five 20% cdr items they would end up with 50% cdr after the soft cap.
both have cdr -.-
I guess I can consider it being pure tanky outside of the passive, but again also wasted stats on cdr.