Okay, challenge accepted. I will write a non-isekai novel with serious fantasy premises that I will focus on and promise not to drop because I got bored again! (Disclaimer, out of my 21 novels, 15 are isekai. 2 LitRPG. 2 sci-fi. 2 Naruto fanfics. 19 are virtually dropped, three finished books in one isekai novel, and one finished book in one superhero scifi novel.)
Don't get me wrong, I loved fantasy as much as I loved isekai. The beautiful thing about isekai is how you put a character's mindset in another world and how it would affect their journey. From there, the character would evolve. I have lots of favorites when it comes to the isekai genre. The problem with JN's popular media with their isekai is how they use salarymen and high school loner teens so much. I reckon, it resonates a lot in their population because of how tough the life of a salaryman is as portrayed in most there stories... And when they do try to be creative, they often go to the extreme end of reincarnating into just about everything. Still, there is a lot of interesting isekai stuff out there, even in SH.
A really helpful advice to Isekai writers is to break the mold and not isekai the same kind of character again and again. A way to do this is to isekai a 'complete' character, because they would quickly become 'incomplete' the second they arrive in the other world, as they were faced with a new environment and rules. By 'complete' character, it could be a Demon King, a Taoist Immortal, a renowned Knight, etc. This had been done in manhwa/manhua, and they are interesting enough for readers to stay the long way and be hooked. An isekai with good worldbuilding is awesome, but it is the MC in the end that will make or break it.
Isekai comes in many flavors, from transmigration, reincarnation, possession, dungeon, reversed isekai, and rarely even regression.
Another tip from a fellow isekai writer... I have this habit of giving every character I watch in popular media the isekai treatment and imagining in my head how these characters would interact with different worlds. Speaking of 'world', you have to treat it like a character so that you don't lose your vision for the story. Anyway, back to the topic at hand. Think of this as more of a mental exercise. I call it imagining a fanfiction in your head. For example, last month, I was watching Blade Runner 2049 with Ryan Gosling, and I thought what happens if we gave his character the isekai treatment, a cop from a cyberpunk civilization. I thought it would be interesting to see him in a steampunk world, even a medieval fantasy, or even a grimdark superhero story! Now, what's next is to think of a method how to send him to another world. Maybe a science experiment gone wrong, getting shot by a bullet, getting dropped by a truck, etc. What's next? Build a motivation for the character, understand his perception and the people's perception in general. From there, the story would snowball as you build one arc to another.