I think Zero No Tsukaima sits on the cusp between old-style isekai and the new-style isekai. It was also one of the fairly popular LN adaptations during the big wave of LN adaptations that happened in the early 2000s, so I'm wondering if it had anything to do with influencing the genre. It doesn't have the explicitly video gamey elements of modern isekai, but it's definitely more similar to them (with its harem-ish antics and the MC getting a magical powerup by being transported to the new world) than to earlier isekai.
One of the things to remember is that most of the successful modern isekai came from the webnovel scene, then got picked up by traditional LN publishers (leading to manga and anime adaptations) once they'd proven themselves popular by crowdvoting on the original webnovel sites (the publishers already knew they were working with winners), so there was a bit of a time lag from when the genre got popular online to when we saw it hit the manga/anime scene. Speaking of popularity, that's one of the reasons you see a lot of the same lowest common denominator crowd-pleasing tropes across them - getting popular enough to get a publishing deal is highly competitive, and it's a much better bet to stick with what everyone knows works. You also see stuff added, removed, or shuffled around at the publisher's/editor's request, based on what they think will do better with the wider LN/manga/anime audience. (For instance, the addition of Albedo in Overlord to give the series a bit more eye candy in the LN release.)
@DANDAN_THE_DANDAN
Come to think of it, what do you guys think about isekai series like Slime Tensei, Shield Hero, and Spider Isekai? I've watched Gigguk's video on isekai and he believes that these are the "next generation" of isekai
It's something you see with any genre once the 'standard' feel is set, writers start putting a deliberate twist on certain elements of it in an attempt to grab more readers who are getting tired of the standard genre, but not so much of a twist that it loses that feel. (Speaking of a grandaddy isekai, A Connecticut Yankee In King Arthur's Court was the same thing, except it was twisting the knightly fantasy genre, stuff like Ivanhoe and Men of Iron, by making the main character a modern engineer instead of a medieval knight.) For isekai, there are some easy twists you can put on the standard formula: protagonist as a monster, protagonist as a villain, protagonist as an underdog people in the world despise, etc., so we're starting to see a lot of works using them, with more following as the various subgenres get popular.
We're seeing it a lot faster in the manga/anime community for isekai now, because new webnovels and LNs are getting picked up for manga/anime adaptations much faster than the first isekais were. The webnovel scene (and the webtoon scene) iterates on ideas a lot faster than some other storytelling scenes because its feedback is very fast and direct, and there's no editorial gatekeeping as there is for traditionally published LNs and manga, so people can get riskier new ideas and twists on the genre out to the public faster.
And while we're at it, what do you guys think about Otome? I think we can all agree that this comes from isekai but what exactly caused the boom?
I think it spiked so suddenly because someone asked "wait a minute, what about writing isekais for people that want romance and drama instead of powerleveling badasses? Why not base them on reverse harem otome games instead of Dragon Quest style RPGs?" and that pulled in an entirely new demographic of readers, while still retaining some of the existing isekai fans. You could say that if the usual isekai presents a male power fantasy, this subgenre presents a female power fantasy.
Although they're called an isekai subgenre, they have much more in common with fantasy shoujo than with the series that made isekai popular originally.
