Warning: I've never written a story but here what can I think of to avoid some mistakes, most are obvious, people always do them, even this sentence is unoriginal and obviously obvious, still, read and use your brain.
mMMm I'd say to call an isekai "good enough" or more, you would need first and foremost to not have the major "isekai focus" you're starting on. You want to write a proper isekai that is original, well-written, and has that special ingredient you're yourself yearning for, but it should not blind you, I don't think that should be the thing you should focus your attention on, otherwise it will only be predictable arcs and bland characters no one wishes for.
I'll start with the isekai setting, how I think it should really be done. The MC gets transported to an other world, sure. Why, how, where, circumstances. For the transmigration, there should be more than just getting isekaied, you don't only get a magic circle out of nowhere or truck-kun then meeting with (a) God (I've read too many of these, I just can't anymore). I don't have many exemples but my favorite isekai by far is The Saga of Tanya the Evil (the books, the anime and manga to me are very different from the original source which really is a pleasure to read). It takes a variation of truck-kun, train-dono, also the 'god' is not a real god, the god can be considered as "evil", he's just inconsiderate of the MC who started a debate with the entity and immediately regretted it. The god is not dumb, neither is the MC, the setting is fantasist but it's well-done, the gods have actually a purpose for sending the MC in another word, testing faith, and you learn a bit later that actually the "god" was in actually not alone, and they stay in the shadows of the humans, allow war, as long as people actually believe in them, they manage each a part of a world and the MC is a sort of experiment on who they bestow curses and misfortunes.
The childhood (not necessary and often brings more bad than good). A 6 years old do not understand philosophy nor politics, at that age you were sleeping and playing, nothing more. Same for 10, you're stupid and blind, you only have flawed views of the world, it just makes no sense that the 10 years old will speak as a teenager/adult would, just stupid and if I were the parents, I would worry about that, not just telling myself "wow my child is a genius, you're so intelligent and proficient at magic". In fantasy/isekai stories I judge good, the childhood is only briefly mentioned in the prologue or first chapter(s). Also obviously that depends if your MC get reincarnated or not, and I've never seen any really good isekai/fantasy with full reincarnation (re-starting as a baby), because if a baby has the mind of a grown-up then obviously it will only bring miseries to the character, and bearing with years of being stuck into a weak body in which you cannot accomplish anything except reading books is hard. I take once again the example of The Saga of Tanya the Evil, her childhood was described as Tanya being Tanya, a quiet but smart kid, not talking with children and solitary, but then I must admit that when she enrolls in the army at 10 years old it's a but unrealistic but it's at least said, the characters actually wonder and fear the entity/demon that is Tanya. She read a lot of books, it can be passed as a genius but she's hardworking, on the surface a war-maniac but is not. She is a complex yet simple character but with logical settings and feelings, it's realistic, because she has actually experienced war herself as a child, she has killed in front of her allies, they know what she has done, she's just a bit extreme but it's plausible!
What I mean is if the character has an isekai childhood, plot holes will come if you're not careful.
Then the rest, either reincarnated as an adult, keeping your adult body or finally grown-up, the character is an adult and should act like one. I understand that Japanese authors wish to write teenagers characters but most teenagers I'm sorry are dumb and do not think, mostly because of modern society that does not emphasize maturity. And so in a sense, their stories are realistic since the decisions that the authors' MC take are often stupid, but I'm often wondering if even their character are somehow smarter than their creator but whatever. You should weight all the decisions your character make but actually think them. The protagonist must think, in advance, what could happen if they did this and that, the consequences, the righteousness if they care about that, not just act on emotions. Being hot-blooded is true but it's bad and we don't want that, we want someone that actually thinks a little and do not take decisions in a hurry, they could keep a journal or just remember to think about certain parameters before actually doing their actions.
I want to come back on what I said in the first paragraph, even though the obvious setting of the story is isekai, if it's just that then we can more or less predict with the world you give what might happen, something you'd better avoid, but it's not going to be an easy task.
If it's a litRPG then get rid of the adventurer guild and the mana-measuring crystal, do something more clever or put it originally somehow. Also "medieval" is important, most of the fantasy sure have kings, lords, nobles and the such but it's not medieval. Medieval is darker, scarier, people are actually poor, to have that you can't just add 3 homeless people in the street and 2 super-rich arrogant nobles, no please no. People are actually intelligent beings, that have more than 1 mask, not "in appearance good but it was a fake smile", sure but there's more to it, I don't think there's a need for a detailed backstory but multi-dimensional character is elemental.
Also, think about everything about the world, not just a map and the currency, how do people live, eat, how is the terrain and resources actually managed, it should not be something that the MC can understand and resolve in 2 days, but way more complicated than that, it's a country we're talking about because he read 3 novels on the net!
As for any story, think about the story ahead, perhaps not the end but I'd say you must have an idea or things you think should happen later on in the story, don't put useless arcs because why not, and no slave harem please. What I would advise is to wait 1 week at least before coming back on what you have written and realize how bad it actually was even though on the moment it seemed godly.
Here is what I can currently think of, I don't have any prior experience to back this off, just some thinking and a critical mind that for me are a must for writing but also reading.
