Someone (sorry, it's been years) had the idea of intersecting power sources to get classes. This was before the actual advent of 4e, so we had just rumors to go on as to the actual sources and how they would work. I played around with the idea, eventually developing an Excel file to help me keep track of everything.
The initial group of sources was Arcane, Cunning, Devotion, Influence, Martial, Mundane, Nature, and Urban. Laid out on a grid, the intersection of two sources was a class.
Arcane + Arcane = Wizard.
Arcane + Cunning = Illusionist.
Arcane + Devotion = Theurge.
Arcane + Influence = Magus.
Arcane + Martial = Spellsword
Arcane + Mundane = Dabbler
Arcane + Nature = Greenbond
Arcane + Urban = Magician
I've found this a useful tool for generating and evaluating class concepts. Not every class maps perfectly into this, and some are downright esoteric and weird, but the benefits outweigh the drawbacks.
OSR games typically don't have that number of sources, but it's interesting to map what they do have. Personal interpretation plays a role, but this was my result:
Races can be mapped as a generic "Racial" concept. Adding a Nature source would make space for the Ranger (Martial/Nature), Druid (Divine/Nature), and Gnome (Nature/Racial). You can also insert the races straight into the generic sources above (Arcane/Martial=Elf, for instance), but there is overlap (Halflings & Assassins, Dwarfs & Fighters). This table adds Racial, Nature, and an alternate Racial that includes Half-Orcs:
Obviously, the addition of new sources increases the diversity. Power sources can be generic, as above, or campaign-specific. Cross the generic sources with "Indian", "Egyptian", or "Norse", and you've got cultural classes that cover the traditional D&D class roles. Sources can also be more specific; my second set of sources is Artifacts, Bargaining, Feudal, Ki, Law, Lore, Primal, and Stealth. Cross these with the first set, and you get this:
My tertiary source list has gotten even more experimental. To be honest, it's mostly blank space. Chaos, Genie, Light, Order, Runic, Psychic, Shadow, and Technological don't always play well, or at least easily (actually, Technological does - definitely a fertile concept).
Here are the Feudal and Technological sources and all their classes:
I worked this idea into my Random Fantasy Class Generator on Abulafia as Class Themes (other people have made additions, so it doesn't match up exactly).