I've always enjoyed the game cookie clicker, so let me thank you for the awesome work you've done.
Anyway, my idea is relatively simple. Implement a mechanic, where each click has a certain chance to give more cookies than usual. I'd say start with a 5% chance for 5x cookies. Something small yet rewarding.
After the basic critical click, you can give upgrades that either increase the chance to get a critical, or upgrade the amount that the Critical click will reward. I have ideas on the intervals when the upgrades should show in retrospect to the cookies made/ being produced, but I'll leave those out for now.
I know that features like this have been used in other games so maybe call it something else besides "critical".
Perhaps something like "Bountiful Click" would be more appealing? Just suggesting it.