The Current CDR System
- Cooldown reduction is a champion statistic that decreases the total cooldown length of each of their basic abilities by a percentage. Cooldown reduction stacks additively and has a hard cap of 40%.
I'm going to be blunt: this is not a well-designed system. Having CDR stack additively gives it increasing returns; going from 0% CDR to 20% CDR gives your abilities 25% more uptime, but going from 20% to 40% gives your abilities 33.3% more uptime. CDR is the only stat in the game that has increasing returns (now that dodge has been removed [don't ask]). This effect is countered by the very low cap of the 40% and the fact that there are no stackable CDR items. CDR os the only stat in the game besides attack speed to have a hard cap, and it's about twice as easy to hit that cap in a standard game, making the player feel that they're wasting gold.
How I'd Remake CDR
- Cooldown reduction is a champion statistic that increases the rate at which each of their basic abilities come off cooldown by a percentage. Cooldown reduction stacks additively and has a hard cap of 65%.
So having 20% CDR means that your abilities refresh at the rate of 1.2 seconds of cooldown per second. This is how all stats should work, increasing values instead of decreasing them. By increasing the refresh rate, CDR now has decreasing returns; going from 0% to 20% gives your abilities 20% more uptime, but going from 20% to 40% gives 16.7% uptime. The 65% cap is there because that's the amount of CDR you'd need to get the same ability uptime you have at 40% CDR using the current system.
First off, small sources of CDR are much less harshly impacted by this change, especially at the beginning of the game. For this reason, I don't think we'd need to change most CDR sources less than 15%. The higher-end CDR items like Nashor's Tooth, Glacial Shroud, and Heimerdinger's UPGRADE!!! would probably get buffed since they're the most strongly impacted. I could see some 15% CDR items getting buffed while others stay the same, but that could be taken on a case-by-case basis. What's more, the possibility is opened for certain CDR items to be made stackable. Maybe a CDR Doran's item?
This system allows for less strict item variety while preventing exploitative builds.