What if you change Critting to "Skill" instead of "Weapon used"...
So...you remove the Threat Range and Crit Modifier from the weapon tables and replace them with :
Base Attack 0-5 you Crit on a 20
Base Attack 6-10 you Crit on a roll of 19-20
Base Attack 11-15 you Crit on a roll of 18-20
Base Attack 16-20 you Crit on a roll of 17-20
Improved Critical feat gives a +2 Crit
This only works with weapons you have proficency in...any other weapon is just a straight 20 all the way.
This is meant as a way to choose a different weapon NOT based on Crit, but for flavor/looks. (Now to find a way to have all weapons deal the same kind of damage, IE all Two-Handed weapons deal 2d6 dam-kind of thing)