The way I'm thinking after considering is doing it like this: If the roll is within 5 of your target, that's a narrow win or loss. 15 more or less than that (ie 6-20 off) and thats a success or failure normal. Anything more or less than that is a big loss or win. This gives you a pretty big margin of error and means that people with real big stats can still fail but not hideously.
In terms of combat, we compare the degrees of failure and success so that even dual failure can end in someone's advantage (sometimes). I'm thinking it should breakdown like this: You deal increasingly more damage depending on the difference between the two rolls. If the two are identical, then whoever either fails by the least or wins by the most relative to their target number is the winner. They deal no damage but gain the advantage the next round. If the rolls are "Abutting" such as a slim win vs a normal win, then 1/2 damage is dealt. If they are 1 apart, such as win vs big win, they deal normal damage. Two apart, they deal 2 times damage and so on. Big win vs big loss deals 4 times damage.
I'm considering how to handle limb severing too. I'm thinking of making it so that the only way to get the limb is to sever it mid fight. Under normal circumstances you swing random and have a 25% sever chance for any limb reduced to 0 hp. You can pick a limb for a -5 disadvantage, and both pick a limb and try to deal severing damage (ie 100% chance of sever if you reduce the limb to 0HP with that attack) for -10.
How weapons work is another conundrum. I'm thinking about having It effected by 3 things: Skill, Stat and Variable. Skill will just be a lock off thing. Need X skill to use. Better weapons need more skill. Second is Stat, and that would mean that your weapon would scale to some degree with your stat. So you could say a weak scaling weapon might have .1 scaling so 50 stat would be +5 damage. Meanwhile a high scale weapon might have .5, which would add +25 to the attack. And Variable which is a particular set of dice to roll that acts as a bit of damage randomization.
So a long sword might be like
Sword skill 20, .1 scaling dex and str 3d4. So at 60 dex, 40 str, you have 3d4+10 damage.
This seems kinda complex but with rounding and the fact that stats don't change often, it will be a fairly straight forward roll. Plus it lets weapons vary and also reflect character build!
Oh, Piecewise is posting more system stuff. I guess it's time for me to complain.
Is 1d100 that much better than 1d20? It just allows for a little bit extra granularity--if you're making changes of five points (yes, I know that was just a random example), then you're essentially making a d20 system with more unnecessary numbers. And that can result in fuzzy weirdness with optimization, like in Dig where you want a Mind stat that's one point above a number divisible by five, because it means you're guaranteed to survive one more turn of being dead. Also, weirdly, 1d100 seems to result in less variety in characters--In Dig, a profession gives you only a 12.5% skill boost in one skill, and in Oro everyone had nearly identical stat averages due to the large number of dice rolled to randomize starting skill. Compare to WIZARDS or NuER, which feel like they have more differentiation between characters.
For the success levels, are they flat tiers of success, as in beating the target by twelve points always results in the same level of success, even if the target was 13 or 113? Meaning low skill people can never hope to get the equivalent of an RTD's 6-1? I'd think not, since experience growth depends upon a "great success", which would mean people with super low skill can never get better. I just can't see how you'd do something else while still keeping it simple.
If experience only comes when you score a great success, does that mean extremely skilled people who succeed by larger margins would snowball their skill?
You have a point with the level up system. It's probably better to just go an ER route and reward level ups upon certain successes.
As per D100 vs d20, the main reason is basically d100 is more obvious to me as I balance (I suck at balancing) and lets me give more bonuses which people are always clamoring for at all times. Where as d20, even a +1 bonus is pretty big. If I remember right in D&D the max possible bonus is +10 and thats at a level attainable only by legendary monsters and demigods. Frankly, I have problems handing out advancements that aren't OP and d100 softens that.
As per success levels, I talked about them more above and hopefully that explains it more . Are you complaining that they're flat when they should be scaling?