Artificer: Artificer, Artillerist, Battle Smith, Armourer
Barbarian: Berserker, Totem Warrior, Ancestral Guardian, Storm Herald
Cleric: Life, Trickery, War, Grave
Druid: Dreams, Stars, Moon, Land
Fighter: Champion, Psi Warrior, Battle Master, Eldritch Knight
Monk: Mercy, Open Hand, Shadow, Elemental
Paladin: Vengeance, Conquest, Devotion, Redemption
Ranger: Beastmaster, Gloomstalker, Hunter, Monster Slayer
Rogue: Arcane Trickster, Assassin, Thief, Mastermind
Sorcerer: Wild Magic, Draconic Bloodline, Aberrant Mind, Shadow Magic
Warlock: Great Old One, Hexblade, Fiend, Archfey
Wizard: Enchantment, Evocation, Illusion, Conjuration
I think it's important to keep basic, vanilla versions of each class for new players, so subclasses like Champion, Thief, and Berserker play an important role. Some classes were hard to cut down to four (Cleric and Wizard!) while others didn't need more than 2-3 (Sorcerer, Monk, Artificer), so I could see a case being made against there being a flat 4 subclasses for each class.
Also, currently, some classes are so dominated by a single subclass that the rest, though cool, are almost superfluous. I would not be shocked to learn that more than 75% of experienced players pick Moon Druids, Mercy Monks, Hexblade Warlocks, etc. So I really hope that 1D&D is able to achieve better balance.