I get that a game like League of Legends is a real pain to balance and since I only started playing towards the end of season 3 I didn't experience a lot of the disgustingly broken champs and builds.
However I am one of those people who sit there pre-lobby theory crafting my builds for champs etc. And I feel like back in season 3/4/5 **building CDR was a choice**, it was a trade off with pro's and cons_ (except Spirit Visage that was super over-statted)_ and often my 'optimal builds'_ (I wasn't high ranked so they were probably dumb)_ would only have 20 or 30% CDR.
Nowadays, **every main Magic Resist item has 10% CDR**_ (It would be nice to have a more purely statted MR item like old banshee's veil considering AP's have the highest % penetration with Void Staff)_, both **Tri-Force and Black Cleaver have 20%** where they used to have only 10%_ (At least one of these two items being built 100% of the time on all Melee AD champs)_. This leads to me feeling like it is impossible to properly optimise builds to 40% CDR unless I want to have 70% with Transcendence and gain adaptive damage _(**which then limits rune optimisation**)_.
I believe reducing the accessibility of CDR would help to solve a few problems such as tanks having too much damage _(due to their high base damage being on low cooldowns)_ as well as certain tanks not feeling tanky _(due to** less resists in items to compensate the cost of CDR**)_ It would help stop bruisers and assassins snowballing after completing their first item even if they are largely behind (this in turn would also slow down the lane phase_ (as you are trying to achieve with turret plating)_. It would bring champs with no mana/energy feel better to play against again due to longer cooldowns so they aren't trading for free as often_ (they could maybe even be buffed)_. I imagine it would also help the balance team to **pinpoint which champs need buffed** rather than a blanket fix like Black Cleaver for instance _(which in my opinion does more damage than good)._
Too many items having too much CDR leading to poorly optimised builds and many champs snowballing far too easily
As I have said I am not a high ranked player so I am probably completely wrong but I thought it would be interesting to think about
_Pls No Flame_