While less than happy about the skill system used in D&DN I did like the following s20 skill systems some more than others. For those of you who are unfamiliar with them I will include a brief description of the way they were run and some subjective cons of each one as none of them were perfect.
The concept of bounded accuracy if tweaked could make some of these hum.
Second ed did not have skills as such but it did use ability checks and non weapon proficiencies. AN optional rule allowed characters with a high intelligece to take extra non weapon proficiencies and a high intelligence warrior type could take extra weapon proficiencies instead. Thieves did have a basic skill system that was later divourced from classes in 3.0.
It was not really a skill system as such and you had to roll under your ability score to pull something off peraps at a penalty if it was difficult.
The 3.5 system used skill points where as you leveled up each class got XYZ amount of skill points you could spend on selecting skills from your class list. A fighter or wizard for example got 2 skill points a level while a Rogue got 8. Bonus skill points were granted for high intelligence, and at level one your skill points were multiplied by 4. Overall id did kind of work.
Some spells obsoleted skill checks (knock) and skill points got fiddly. There was also a huge extreme between classes that got 2 skill points and others that got 8 skill points. Some skills were also better off consolidated.
Star Wars Saga Edition.
A personal favourite of mine the basic system was half your level and add 5 if you were trained in a skill essentially identical to 4th eds system. Due to a change in genre new skills were added (use computer, pilot, use the force) and skills were consolidated from 3.5 (Stealth, Perception, Deception etc). Bonus skills for high intelligence were granted. Skills were integrated into class abilities and races to a greater extent than other d20 games. Probably the best non combat system on this list.
Jedi were the only class that had 2 trained skills (3-6 should have been the range IMHO). Skill focus use the force was very silly as skill checks were used for force powers. There was also a very large gap between someone who was trained/specialised and someone who was not. At level 1 for example being trained and having skill focus gave you a +10 to a skill check (+ ability score mod) while some races also had a +5 racial bonus (+15 lvl 1). Perception was a no brainer skill (spot, listen, search, and sense motive rolled into 1). Starting as a high skill class and being trained in lots of skills and then multiclassing was rewarded mechanically.
4th ed had a very good skill system for the most part despite somethings like skill challenges maybe not working as intended. It used the same system as Star Wars Saga (half level +5 if trained). 4th ed characters did not gain bonus skills for being intelligent. Some utility powers granted bonuses to skill checks as well. Backgrounds and themes were used which were interesting. Overall 4th ed had a very good skill system that may not have been implemented that well in places (subjective though).
Similar to Saga regarding specialisation but with less problems caused via racial bonuses. Every class had some skills pre selected which in most cases you would want anyway but not all of the time. Every rogue for example has thievery trained regardless of your back ground. Could have used a few more skills IMHO and the non combat aspects of 4th ed were overshadowed by the power structure of the game.
Resembles 3.5 at first glance with skill points (ok skill ranks, same thing really) but they have been fine tuned. Each class has XYZ aount of skills and when you spend a skill point on a skill you gain a +3 bonus for being trained in it. Skills have been consolidated and traits (back grounds really) can give you a bonus to skills and access to skills not normally on your class list. Alot of the traits have also been tied to the Pathfinder campaign world. Some classes also have skill use tied to class abilites.Bards for exampe get to add half their evel to knowledge skills and use perform to duplicate other skills.
Someof the new skills are just silly/to situational (fly). Perception is a super skill although not quite as bad as Star Wars Saga mostly due to every class in Saga having access to it. Some people may also not like PF retention of skill points. Otherwise PF has siilar problems with skills as 3.5.
In conclusion this is a brief summary of the last 4 versions of D&D and Star Wars Saga and the way they have dealt with skills. All of them have their pros and cons. 3 of them are what I consider "good" at least in the way they run things. Second and and 3.5 have either not aged well or the other 3 havejust doe things better IMHO. 4th Ed/Saga are very similar in skill use the main differences cming down to genre and class construction while Pathfinder uses a refined 3.5 system.
Personally I would like a refined Saga/4th ed skill system for D&DN. Make the numbers smaller (half level +3 for being trained, skill focus grants advantage maybe). Make intellignece more than a dump stat for everyone who is not a wizard. Some classes should also have more skils than others classes but they seem to be rewarding Rogue types alot more than every other class as its back to the I have twice as many skills as every other class a'la 3.XYZ. At least its not X4 skills (2 vs 8) of 3rd ed I suppose. 4-8 skills should be the range for most classes (or 3-6 if skills are consolidated like Saga/4th ed)
I'm not to worried about skills being tied to class if back grounds/themes/traits etc let you gain access to skills one normally would not have access to.