@Weasalopes said in The Originals:
As in, the first work introducing each theme?
I suspect that you might need to settle for the first several, given the question of who first thought of it and who first started posting their WN.
And you would need to go back to the web fiction in a number of cases, given that the first instance of a theme in online fiction may never actually get published professionally.There'd also be the question of those themes that have also shown up outside of the Light Novel environment prior to appearing inside of it.
When you consider the pulp SF from the early to mid 1900s in the US, a number of those showed up there considerably before Light Novels were a concept. And even there, a bunch of the ideas were taken from still earlier works...
Just about every time travel romance as well. Super popular in the 90s and they were a dime a dozen. Outlander pretty much has time highways but at least those have their consequences. Some authors treat it like a doorway.
Being in an actual video game too like Sword Art Online: User Unfriendly published in the 80s. (Can't remember which author wrote it I haven't read it in more than 20 years.