I've always felt that the hit roll and weapon damage should be linked. A really good hit should do more damage and a barely successful hit should less damage, so here is what I'm thinking. This applies to the raw roll without any modifiers and only when the roll is a hit of course.
20 A critical, double damage
15-19 Maximum damage
10-14 Average damage
05-09 Minimum damage
For example:
Thug's mace: Max is 8, Ave is 5, Min is 3. Easy enough.
Thug's Heavy Crossbow: Max is 10, Ave is 5, Min is 1.
Since Roll20 automatically calculates weapon damage for the players this will only be done for the foes but I think it should work.
My only question is would the critical be double the max damage or double the average? I'll have to think about that.