I would really love to see more subclass options added into the game, specifically the subclass options that have been officially published by Wizards of the Coast outside of the PHB, but not the subclass options that appear in books meant for other realms (since the game takes place on the Sword Coast, in The Forgotten Realms). So the ones featured in The Dungeon Master's Guide, The Sword Coast Adventurer's Guide, Xanathar's Guide to Everything, and their new book, Tasha's Cauldron of Everything (yet to be released). Please, please, please add these in.

Here is a list:
Barbarian
- Path of the Ancestral Guardian *
- Path of the Battlerager (Dwarves only) *
- Path of the Storm Herald *
- Path of the Zealot *
Bard
- College of Glamour *
- College of Swords *
- College of Whispers *
Cleric
- Arcana Domain *
- Death Domain *
- Forge Domain *
- Grave Domain *
Druid
- Circle of Dreams *
- Circle of the Shepherd *
Fighter
- Arcane Archer *
- Cavalier *
- Purple Dragon Knight *
- Samurai *
Monk
- Way of the Drunken Master *
- Way of the Kensei *
- Way of the Long Death *
- Way of the Sun Soul *
Paladin
- Oath of Conquest *
- Oath of the Crown *
- Oath of Redemption *
- Oathbreaker *
Ranger
- Gloom Stalker *
- Horizon Walker *
- Monster Slayer *
Rogue
- Inquisitive *
- Mastermind *
- Scout *
- Swashbuckler *
Sorcerer
- Divine Soul *
- Shadow Magic *
- Storm Sorcery *
Warlock
- The Celestial *
- The Hexblade *
- The Undying *
Wizard
- Bladsinging (Elves/Half-Elves only) *
- War Magic *
* Sword Coast Adventurer's Guide* Xanathar's Guide to Everything* Dungeon Master's GuideI actually think I would be fine with not having the Sword Coast options (even though it fits), but the Xanathar and DMG options I feel like are a must.