My group and I were finding that the assurance feat seemed too situational to use most the time, and usually only with a few skills. I had the idea for a minor alteration, but wanted to see if it would be too strong or not.
I would changed the feat to when assurance is declared on the action, you use the 10+proficiency bonus as the minimum result, but cannot score a critical result on the check unless the minimum would have resulted in a crit.