I like this with the caveat that bonuses to the first attack and only the first attack do not count for determining if you can critically miss later in that round. True Strike being an example of a bonus that only applies to the first attack of the round.
As I sit here contemplating how this may fall apart I think we have a couple things to consider.
I'm not sure how opponent AC scales with character AB - if a typical encounter for a 10th level fighter is a creature with AC 25, this modification won't make a difference.
High attack bonus does not necessarily equal skill. Haas can jack up his attack bonus for an encounter by pumping up his Strength to obscene levels. Just because he has a 50+ strength and a corresponding attack bonus in the 30's doesn't mean he is a skilled combatant.
Should the target's AC factor in to how big of an error the attacker made? Target AC already established that the attack was a miss. Seems to me that attacker skill is what would keep a regular miss from turning into something disasterous.
Perhaps using just BAB vs a fixed DC (independent of opponent AC) would be better as an indicator of skill to avoid a critical fumble. You could effectively treat it like a BAB-based skill check (e.g., d20+BAB vs DC20). That way low level characters can avoid a critical fumble if they get lucky, and skillet warriors can skill fail if they are especially unlucky.