I've been working away at the concept of class overlays for a few weeks, though admittedly my work has been pretty sparse.
I had to scrap a number of them because they were getting to the point where they were effectively separate classes, which defeats the point of White Box FMAG, in my estimation.
Here's the level of complexity I'm aiming for:
Templars/Temple Knights (Cleric Overlay)
*Alignment: Must be Lawful, there are no Chaotic Templars
*Weapons/Armor: Add the longsword and crossbow to the list of available weapons
*Spellcasting: Templars cast spells as a Cleric two levels lower, meaning they do not get a spell until 4th level.
*Turning: Templars turn undead as a Cleric two levels higher. Templars can also use this ability to turn demons/devils, though a "D" result does not destroy these creatures, but rather banishes them back to wherever they came from for a year and a day.
*Templars gain +1extra hit point at each level.
Everything not specifically mentioned above is the same as the regular Cleric class.
Or my earlier draft of the Witch (Magic-User, obviously)
*Spellcasting: Opponents save at -2 vs. Witch's illusions and charms. Witch calculates duration and range of illusions/charms as if 2 levels higher. Can learn and use Cure Light Wounds as 1st level Magic-User spell. Can't use direct damage spells like Fireball.
*Can brew potions at 5th level
I had a Druid write-up, but it was almost just a separate class. I also have an adaptation of the Acrobat class written up by JB in his Complete B/X Adventurer, which is not as complicated as the changes from Cleric to Druid but still might be more like a second class. Here:
Acrobat (Thief Overlay)
*Weapons: Can only use staff, dagger, hurled weapons
*Armor: cannot use any
*Thievery: Now called Prowess, can be used for physical stunts. scale sheer surfaces, balance, escape from melee combat at full speed, climb overhangs, pole-vault, reduce damage from falls, etc.
Simple changes, mostly a different class, one could argue.
Consider the Druid:
*Alignment: Can only be Neutral
*Weapons: Staff, dagger, sickle (treat as hand axe that cannot be thrown), sling
*Armor: Leather only, no shield
*Spellcasting: Can't cast spells with Law/Chaos in the title, can cast Polymorph as 3rd level Cleric spell (but only on self, and only into natural animals) Can cast Plant Growth as 4th level Cleric spell.
*Turn Undead: Cannot turn undead, but can turn animals/beasts (and plant monsters at DM's discretion) Result of "D" does not destroy, but Druid can keep a companion as with Chaotic Clerics and undead.
So those are the ends of the complexity spectrum I'm looking at. I'd really like to make my overlays newer things and not roll out the same cast of classes that I've seen over and over again in every edition of D&D.
I'll continue to work on this. It isn't my main focus right now, but it's a fun little thought exercise.