This is one of those ideas so simple, I'm certain someone else came up with it. Maybe I read it and forgot about it.
So in many D&D versions, both official and house-ruled, there's this weird thing when you roll one number to see if you hit the orc and another to see if you chop its fool head off. This means you can end up with a range like this:
1 ... Miss! (maybe it's a fumble if the DM is cruel)
2-11 ... Miss!
12-17... I hit! Alright!
18-19 ... Argh! So close to a critical!
20 ... Yeah, baby! This orc is toast!
That 18-19 range is my concern today, or really any number that's less than a critical but well in excess of the minimum needed to hit. Tom Moldvay tried to smooth out the transition from normal hit to uber crit by adding in a chart of extra effects. What I'm about to propose here gets you an extra effect for a better hit while dispensing with the rigamarole of tracking who is carrying what penalties to their actions.
In BX all standard weapons do d4, d6, d8 or d10. That's a 2 point jump in max damage for every die size increase. So here's my basic idea: for every 2 points you exceed to to-hit target, bump your die up one size. If applied to the monsters as well that's going to make it easier for high hit die monsters to mangle the PCs.
Before trying this one would need to set the maximum die size. You could cap it at the largest die actually in use for weaponry, the d10. That means under the right circumstances a dagger is as lethal as a polearm, but nothing is actually more deadly than the normal standards of BX play. Alternatively, you could use this rule as a low-complexity, high-damage alternative to crit rolls by allowing the max die size to get ridiculous. There are many more possibilities once you decide you're not bound by the notions of propriety held by mere mortals:
- d10 becomes d12 becomes d20
- d10 becomes d12 becomes d20 becomes d30
- d10 becomes d12 becomes d20 becomes d30 becomes d100
Or use some funky dice like the DCC rpg folks. The sequence d10 -> d12 -> d14 -> d16 follows the +2 max damage progression, but you could always tack on -> d20 -> d24 -> d30 -> d100 for extra insanity.
Easy to implement (if you can subtract the actual roll from the target roll) and your players will love it (until it kills them).