Well if there are going by the 5e player's handbook here is what classes and subclasses we should be getting in BG3
Barbarian
* Berserker
* Totem Warrior
Bard
* Lore
* Valor
Cleric
* Knowledge
* Life
* Light
* Nature
* Tempest
* Trickery
* War
Druid
* Land
* Moon
Fighter
* Champion
* Battle Master
* Eldritch Knight
Monk
* Open Hand
* Shadow
* Four Elements
Paladin
* Devotion
* Ancients
* Vengeance
Ranger
* Hunter
* Beast Master
Rogue
* Thief
* Assassin
* Arcane Trickster
Sorcerer
* Draconic Bloodline
* Wild Magic
Warlock
* Archfey
* Fiend
* Great Old One
Wizard
* Abjuration
* Conjuration
* Divination
* Enchantment
* Evocation
* Illusion
* Necromancy
* Transmutation
There did say that there will be using other 5e books so we might be getting other classes and subclasses but this is what's we are probably getting in BG3
