I am just curious what people think of as core classes. I tried to cover a wide range of archetypes but I am sure I missed your favorite, so add it in the comments.
If I were to ever create my own D&D Heartbreaker, I would go with six classes, one for each ability score: Fighter (Str), Thief (Dex), Ranger (Con), Wizard (Int), Priest (Wis) and Commander (Cha). I would use talent trees feats or whatever to differentiate characters. I would absolutely NOT do sub classes. I would rather make a game with 50 core clases than use subclasses.
Fighter. Wizard. Scout (as in a skill-based scout).
All other classes in D&D have always been minor variations of those three.
Priest is a divine wizard. Druid is a primal wizard. Cleric is 2/3 priest, 1/3 fighter. Paladin is 1/3 priest, 2/3 fighter. Ranger is a nature-based scout. Thief & rogue are city-based scouts. Barbarian and monk are fighter variants. Bard is fighter, magic-user, thief. Etc.
Everything else is minor variations or specializations thereof.