Design Notes:
Here's the first expansion. It replaces the m20 abilities and skills with a slightly more detailed set, while still keeping it streamlined. This allows for a completely classless system, one that makes it possibly to reproduce any D&D class from a few component parts. One key is Charisma. By having 4 abilities and 4 primary skills, each skill has an associated ability. The four abilities and skills each map to one central game task. This makes it very easy for a player to identify what they want their character to do during the game, and to pick the "right" skills.

EDIT: I realized the original math resulted in lower scores than I expected, so I made a significant tweak to the skills earned per level. I also removed the Perform skill and replaced it with Craft.

Legendary Abilities

At first level, put 6 points in Abilities. One can be lowered to -1 to put an extra point in another ability. 0 is average, 4 is maximum.

Abilities:
Strength (Str) - musclepower, fitness, health
Dexterity (Dex)- agility, accuracy, speed
Intelligence (Int) - mental ability, memory, creativity
Charisma (Cha) - force of personality, leadership, willpower

At first level, put 2 points in Primary skills.

Primary Skills:
Fighting - combat skill, offense and defense
Subterfuge - stealth, sneaking, dirty tricks and thievery
Knowledge - science, magic, history and lore
Social - understanding and dealing with people, art, emotions and intuition

At first level, put 2 points in Secondary Skills.

Secondary Skills:
Athletics - jumping, climbing, swimming, running
Survival - wilderness skills like tracking, hunting, finding shelter
Stealing - picking pockets, sleight of hand
Tinkering - disarming traps, opening locks, building traps and machines
Search - finding or noticing hidden or inobvious things
Healing - treating wounds, poisoning or disease with bandages and medicine
Persuasion - making other people believe lies, including disguises, bluffs, misdirection and salesmanship. Can be used for artistic performances.
Craft - building, fixing, or appraising manufactured goods.

When a character gains a level, they get 2 more points in a Primary skill, and 2 more points in a secondary skill. No skill can ever have more points than the character's level +1.

Background skills: you can allow each character to have a background career, such as Blacksmith or Sailor, which functions as a level 1 skill. The player can add points to it when increasing their level as if it was a Secondary skill.

Non-listed skill: a character can have a Secondary Skill not on the list, if it is not covered by any existing skill. The DM must approve the skill.

High skill option: if you prefer characters to always have some basic level of skill, starting at 3rd level raise all skills with no points by 1 point, and do so again every odd level.

Magic: if using this without Legendary Magic, use Knowledge+Intelligence for arcane (wizard) spells, and Social+Charisma for divine (cleric) spells.