If I was going to simplify things, I would remove the to-hit and damage save rolls. Make everything HP.
You take 1d20+weapon+Str damage.
Or 1d20+6d8 fireball damage.
AC is now temporary hit points at the start of each battle (per level).
Still need saves for Charm and such. But that would speed things up.
I'm in favor of fixed damage, but increasing on how much is over AC.
that way we keep the current AC mechanics and keep crit mechanics somewhat same.
I.E. greatsword is 7 damage + STR mod + whatever.
Roll attack, if you hit AC, deal normal damage,
if you hit 5 more than AC, deal +50% damage
if you hit 10 more than AC, deal +100% damage(critical hit for game mechanics)
if you hit 15 more than AC, deal +150% damage
if you hit 20 more than AC, deal +200% damage(max damage)
graze global mechanics as an option:
if you miss by 5 or less, deal only 50% base damage and cannot trigger any on-hit mechanics
graze weapon mastery:
you graze in you miss by 10 or less, not by 5 or less.
then with 20 STR you can have table:
HIT: 12 damage
5 over AC: 18 damage
10 over AC(critical): 24 damage
15 over AC(critical): 30 damage
20 over AC(critical): 36 damage
Graze: 6 damage