A long time ago (1995) TSR released the Players Option books. To this day they are widely derided on the grognard sites but perhaps they would have been better off being called the DMs Options books. They were.
Skills and Powers
Combat and Tactics
Spells and Magic
High Level Campaigns (DM option book).
Several of the ideas added to these books were later incorporated into 3rd ed. The epic spell casting system in 3rd ed had its origins in Spells and Magic (true dweomers) and Skills and Powers had a form of skill points (character points) that were used in 3rd ed and Combat and Tactics had rules for grid based combat. The main problem they had was that if you let them into the game as written it was very easy to break the game. Parts of them were actually decent enough and I will use the Cleric as an example of modularity. Put simple skills and powers let you build your own cleric. Every cleric got this for free,
d8 hit dice.
cleric weapons and armor
priest spell casting
cleric xp table
priest saving throws
priest weapon and non weapon proficiency.
You received 125 character points to build your priest with. Back then priests did not have a unified spell list although clerics did have a fixed amount of spheres they could cast from. Combat sphere had spells like flame strike, healing sphere had cure spells, plant sphere had entangle etc. Clerics and Druids had different spheres and specialty priests once again had different ones as well along with weapons, armor and granted powers etc. Major access was spells of level 1-7, minor access was spells of level 1-3.
Anyway some basic builds showing how this could be abused.
8 wizard schools 120 character points (15 points per school)
Minor access healing.
This gave you all the wizard spells, and you could cast in armor, did not require a spell book, you leveled faster and you got bonus spells for high wisdom- wizards did not get bonus spells for high intelligence. And you could also cast some healing spells. 30 points could get you alteration and evocation (the best schools as a general rule) and then the other 90 points could be spent on priest spheres.
Cleric (A better fighter).
d10 hit dice (10 points)
warrior priest (10 points) warrior strength and con advantages.
favored weapon. Deities favored weapon including edged weapons (5 points)
Weapon specialization (15 points)
That is 40-/125 points spent. With the 85 points left over one could buy priest spheres and wizards spells. I want to throw some fireballs.
Wizard School: Evocation (15 points) 70 points left over.
Turn Undead (10)
125 points spent.
My character can now cast Druid spells, functions almost as well as a fighter, can heal, raise dead and throw fireballs while wearing any armor and using longswords or whatever. And I can turn undead. And this is one reason why this book is considered bad. It is not actually a bad tool for DMs to use to create NPCs and specialty priests hence my earlier comment about these books being better named as DM options. I'm not that much of a fan of options in the core books, not that worried what they put in a splat but maybe some heartache could have been avoided in 3rd and 4th ed if they ignored the splats that were baked into the core of those books.
Put simply would people want a book (module lol) that lets you build your own classes similar to Skills and Powers or the 2nd ed DMG? This approach would not be universally popular and could be abused like my examples but some people might like that sort of thing. The execution in these books may have been off but IDK about the concept.
Apologies for the ramble but not everyopne here is familiar with AD&D so I put in a basic Skills and Powers 101.