Solving the ADC problem with crit damage rework, and a rune for assassins?

ADCs are supposed to be doing reliable damage, not spiking wildly with crits. What if the adc meta worked like this:

• Critical damage is applied over 3 seconds, and only stacks per champion dealing the crit damage. E.g. if Vayne hits you with two crit cutos, the second crit refreshes the first, dealing the first attack's crit damage if it is higher until the first crit's duration no longer persists. If Vayne and Master Yi both crit on you, both of their crit damage stack together.

• Rune (Precision?): 25% of your critical damage is applied instantly. Takedowns and large monster kills increase this bonus: 3% per enemy champion takedown, and 5% per large monster kill. The maximum is 100%.

If anyone has any other suggestions, feel free to comment.