Diminishing Return on CC
CC would scale down as more CC is added.
It would look about like this.
First stun does 100% duration
Second stun within 5 seconds does has 75% duration
Third stun within the next 5 seconds does 50% duration
Take Pantheon (w), Morgana (q), and Sona (r)
1s (100%) 3s (75%) 1.5 (50%)
Instead of being stunned for 4.5 seconds you would be stunned for 4 seconds. It is not a huge change but it would help enough to see a difference.