With the help of friends and a few play tests I decided to revamp my ability system again. The whole thing started when I wanted new classes that are better things other than spells, praying, and combat. I labeled them as Rogues and they included claseses like Burglar, Mountebank, Thug, Merchant Adventures, and the Claws of Kali (assassins).
I made a whole list of things that OD&D characters can do and called them abilities. Climbing walls, fast talking your way out of a situation, haggling over a item and so on. The way I recommend these abilities work is that you describe or roleplay what you are doing and the referee uses the roll and modifiers as a guideline for the chance of success. Any character can use these abilities but Rogues are better at them then other characters at the cost of either combat and/or spell casting.
Originally I tried making a 3d6 + attribute system work. While it worked well in playtesting the disparity in attributes proved it's downfall, couple with actually trying explain it to other GMs. Most had a negative reaction. The disparity in attribute swamped the effect of the Rouge classes which defeated the point of doing this.
I still want a roll high so percentages are out. So, sigh, I just fall back to using the d20. So now as go through the individual abilities (climb walls, pick locks, haggle, etc) I use this as a template.
If your attribute is 8 or lower you get a -1 to the roll, 13 or higher +1 to the roll. The attribute used depends on the ability. Str or Dex can be used for climbing, Charisma for haggling. To see if you succeed you roll a d20+modifiers and see if you beat a target. I set abilities to a base between 10 and 20. For example trying to climb a steep slope in the middle of combat is a 10, a sheer wall is a 15, and ice covered cliff a 20. A Burglar would start out with a +2 to climbing going up to +8 around 10th level.
I am still working the numbers basically going through a variety of Judges Guild stuff and other older sources and seeing what various DM did. Most of it is in percentages (perhaps the first "universal" task dice) so converted it over to a d20 roll by dividing by 5. Judges Guild's Dungeon Tac Card are a goldmine for this stuff.
I didn't copy over d20 system even though I am using the OGL. This is because I am using Swords & Wizardry as the basis for my project. In S&W (and OD&D) the effect of high (and low) scores are a lot less than later additions.
Along with this and the just completed editing I am ready to layout out my project. I should have an announcement with details this month.