As noted here:
Where d20 breaks down is when it shifts to non-combat rolls where the entire task (skill) is handled with a single, linear-odds roll.
Here’s how to fix that.
There’s a little known optional combat rule that states that people can choose to ‘roll’ their AC every round. Basically, you don’t have a Base 10 AC… figure out whatever you’ve got over 10, call that your “AC Bonus”, and add that to a d20 roll every time you’re attacked.
I doubt anyone does that — hell, I doubt anyone knows it’s there — but look in the DMG.
ANYWAY: while I don’t recommend it for combat necessarily, I think it would be useful for Skill Checks. Many of these are Opposed Rolls anyway — this little house-rule would make all Skill checks opposed.
Find the current DC for a skill. Subtract 10. Whatever’s left over is the DC Bonus. When someone tries to do something to overcome that challenge, the GM rolls a d20 and adds that DC bonus.
What does this do? Two main things.
1. Creates a pyamidal instead of linear success curve. In the case of a Thief with Open Locks +15 vs. a Lock with a DC bonus of +15 (formerly a DC 25 lock):
00.25% You Crit succeed, it crit fails.
02.50% You Crit, it fails
49.5% You succeed, it fails (or, you tie each other)
44.75% You fail, it succeeds.
02.50% You fail, it crit succeeds
00.25% You crit fail, it crit succeeds
00.25% Mutual Fumble
It’s a pyramid curve, but it’s a curve.
2. Removes instances of “Ugh… I got a 19… I know that d20 modules always set the DC’s in five-point increments on everything, so I’ll spend an Action Dice to give me a boost… worse case scenario, I get to a 20, and maybe I’ll get to the 25 break point.” (Particularly annoying on Gather Information charts, when adding the AD will almost certainly glean more info)… If the DC’s are d20+5, d20+10, d20+15, et cetera instead of 15, 20, 25… there wouldn’t be those artifical ‘rungs’ in the DCs to shoot for… that d20+5 DC might be, on your try, a net DC 6 all the way up to a net DC 25… every NPC you talk to is talkative in different ways, after all.
As noted here: