Slayer of Keraptis
I don’t think monks are a variant of fighters. They are completely different. What makes a fighter a fighter? The ability to wear armor and use all weapons. Things monks can’t really do. I think monks are unique as much as any other class is.Thematically, almost every class are variations on the Core 4. Barbarian, Monk, and Warlord are all types of Fighters. Druids are Nature Clerics. Sorcerers and Warlocks are variants on the Wizard (the original magic user). Bards, Paladins, and Rangers are variants of Rogue/Wizard, Fighter/Cleric, and Fighter/Rogue/Cleric. Pretty much only the Artificer and Mystic are actual new themes, and even those are tenuous (not to mention neither is really appropriate for all campaign worlds).