[Discussion] A way to evaluate an Isekai series. Just remove the MC.
Someone I know once said this: "If you remove the main character from an isekai story and its world can still function, that's a good isekai." I think this is pretty spot on. A lot of isekai series are written with way too much focus on the MC, to the point where the world is built solely as a stage for them. This results in bland side characters, one-dimensional and somewhat stupid villains, and other major characters who only act in response to whatever the main protagonist does.
Take everyone's favorite isekai slop, In Another World With My Smartphone, as a simple example. Kick the MC out, and what’s left of that world? Just a generic world where the rest of the cast—both major and minor characters, as well as the villains—loses almost all motivation to act.
On the other hand, good isekai stories usually throw it right in your face what other characters would be doing without the MC. For instance, in Re:Zero, even without Subaru, Emilia would still participate in the Royal Selection, and the whole plot involving the witches and mages could very well play out normally, even if the outcomes might change.
However, this doesn't mean an isekai is completely terrible if it doesn't have world-building like that—you can still totally enjoy Smartphone if it's your cup of tea. But this test easily shows the effort and brainpower put into the story. Only the best writers put in enough details, big and small, for the world to function independently of the main character.
Additionally, I think if you evaluate whether the story remains interesting after removing the MC, you can easily rank different isekai series against each other.
In my opinion, using this metric makes Mushoku Tensei truly the best, even without Rudeus and his endless horny thoughts. Paul would still smash Lilia, Norn would still feel inferior to Aisha, and characters like Sara, Elinalise, Zanoba, and Cliff would still pursue their own goals, which you can easily picture throughout the story's timeline. This applies especially to Rudeus's three wives: even though they are closely tied to the MC, you can still easily imagine Roxy going on adventures and eventually settling down to teach. Eris would still train with her sword and act recklessly. As for Sylphy—who might be the hardest to imagine—her story would still revolve around her friends and dealing with bullying. On top of that, the conflict between certain gods would remain fascinating, even without Rudeus in that world.
Ultimately, as I mentioned above, failing this metric doesn't necessarily mean an isekai is complete trash. But if a story passes it, it is definitely worth reading because it reflects the author's true dedication to crafting the world and its characters.