To be noted: these are to fit within reasonable lore-based bounds, if I had to name some potential unlockable race/class combos it would be:
Alliance
Human Hunter
Dwarf Shaman (if no shaman, then Mage)
Night Elf Mage
Gnome Priest
Horde
Orc Mage
Undead Paladin (if no paladin, hunter)
Tauren Paladin (if no paladin, priest)
Troll Warlock
As you see, I have also offered alternatives for some races assuming Blizzard doesn’t want to go down the path of faction homogenization by introducing shamans to the Alliance and paladins to the Horde.
As per customization options, these could unlock through questchains, similar to how the Eredar Draenei options were unlocked in Retail. There are plenty of NPC skins in the game that are not used for player characters (forest troll, stone dwarf, blackrock orc, the list goes on and on) that could have massive lore-based questchains attached to unlocking them.
Also, hear me out, barber shop.