Isekai (e.g. wish fulfillment + power fantasy) is absurdly popular in the anime/manga industry so I guess I'm rather unsurprised that its leaking over to more traditional novels as well.
As an anime fan, I wish there were more series that strayed from this formula. So many are either isekai, "protagonist has an ultimate power/entity inside them they can't fully control", or both.
You mean like Fieran? That is something author Blaise Corvin calls “exploration fantasy” and inspired him into attempting a new serial along that line.
There is also Your Lie In April. Not a fantasy or reincarnation. There are few other exceptional ones here and there.
Funnily enough I was actually going to write "Frieren is the only one that comes to mind that doesn't follow this formula", but I figured it would sound way too myopic since surely that can't be the only one.
Honestly I was just thinking about this - there have been many manga series (which get turned into anime eventually), that started out with an interesting premise that can be taken into many directions, only to turn into a cookie-cutter shonen battle manga.
I think I know the reason for this - shonen is a relentless popularity contest, where magazines only feature the most popular works and will quickly drop your series once it starts losing readers.
The way to consistently stay on top is to appeal to the biggest demographic - that is shonen battle/isekai fans by making your stuff to be exactly like the other stuff.
But doing so makes you lose the readers who were interested in the thing that made your manga unique in the first place. And after a time, these people will stop reading the publication altogether, skewing fan demographics even more towards genre fiction, making it even more impossible to create something unique.
Anime/manga are pretty strongly intertwined with (web) novels in Japan. Those same novels are fan translated into English (Chinese and Korean too). This has created a kind of fanbase for web novels/self-published novels.
Japanese companies starting to send copyright claims against fan translations probably opened the door for Chinese, Korean, and English web novels like that.
