Got bored had some shower thoughts so decided to do a thread like this...a few fun ideas to explain added classes for races since Blizz plans to expand further...
Let's get the 3 big ones out of the way
Evoker: After an expansion or 2 Dractyr now more familiar with the other races will be able to have a visage of said races. Choosing..lets say a Dwarf Evoker will give you all the Dractyr racials but give you a Dwarf Visage
Demon Hunter: I think the best way to approach it is looking through past events. Illidans rejection of the light can show that fel is extra effective against the light thus opening up the illidari to the rest of the alliance and horde for that expansion much like Shadowlands opened up Death Knights
Druids are probably the toughest sell...for the most part I can argue their are limited mostly due to technical reasons over lore ones. We've seen Nightborne, Blood Elf, and Orc Druids in lore and can assume druids of other races EXIST but are exceedingly rare so for the most part that is going to be the big explanation for most races and I'm gonna stick to it.
The rest will go by Race
Human
Shaman: continued exposure to the Kul Tirans have allowed humans of Stormwind to learn their ways thus Human Tidecallers are a thing
Gnome/Mechagnome: Effectively the same thing
Shaman: learn from the dwarves due to some gnomes desire to harness lightning for power...I mean it's just a good a reason as why Goblin can be Shamans
Paladin: During Turalyon's reign expansion of Paladin's exploded in the Alliance as such gnomes have picked up the mantle...some by force (Lightbound gnome skin included)
Night Elves
Warlock: operating in secret during Turalyon's reign some night elves started experimenting with fel again to combat
Paladin: see gnomes but can also add a thing where a sort of Moon Walker (name tentative) faction opens up as paladins of Elune
Shamans: we already have Night Elf Shamans in Dragonflight as the whatever twilight hammer rip off it's called now just incorporate them in
Dranaei
Warlock: best way I can think is introducing Eredar survivors into the mix and they will still be warlocks and we get a neat red dranaei skin
Worgen
Shaman: see Humans
Monk: they legitimately only don't have monks cause Blizz didn't want to explain how a pandaran was in Gilnaes and Kezan. Exiles reach solves problem
Paladins: Probably would be the most willing to take up the mantle with Turalyon's rule....
LF Dranaei
Monk, Lock, Shaman, etc etc: all would be the same the idea being more exodarian dranaei and the Eredar would be able to go through the Lightforging process extra points for warlock demons also being lightforged
Void Elf
Paladin: would be a little tricky...best I could do is potential issues caused him to Lightforge a few of them into..which causes an interesting side effect that they retain some of their darker energies but the whispers go away. It's an asspull but it's the best I got
Shaman: previous old god relations to the elements allow the void elves to tap into the chaotic energies....easy least
Dwarves omitted because they only have Druids Demon Hunters and Evokers missing and I already explained
Orc/Maghar
Paladin: after the Lightbound expansion the previous orcs were freed and retain their old experiences...we can say some of our orcs get captured and forced this way
(Maghar) Warlocks: Fel is found to be a could counter to the light and thus Maghar give fel a try
Tauren
Warlocks: let's say after this expansion the Grim Totem return to the Tauren on good terms and bring their locks with them
Darkspear Troll
Paladin: Learn from the Zandalari...easy peasy copy paste
Forsaken
Shaman: we have an undead Tidecaller now...why not more
Paladin: Thanks to Calia we now know light based undead are a thing thus allowing for them to be Paladins
Goblins
Monk: see Worgen
Paladin: let's have fun with this one. A big goblin church worshipping the one thing that truely matters (capitalism) is enough for the light to be like "yeah sure" and thus The Order of the Golden Hand rises and Goblin Paladins are a thing
Highmountain
Paladin: they can be Sunwalkers
Warlocks: Couple of Rogue HM learn from the Grim Totem
Zandalari
Warlock: The Reincarnated Hi'Reek and Spider I can't be assed to look up bring warlocks back
Vulpera
Paladin: You know what they join the Goblins venture
Pandaran
Paladin: prolonged exposure to different cultures allow for them to learn the Light
Warlock: Same as the above
Dractyr
able to do the others after continuous exposure to them. Can still use all racials but can fight fully in Visage form