Isekai. It almost feels like a fetish to me right now - I'm almost not interested in reading anything else at the moment. Even the isekai that are technically Sci-fi (trapped in a video game) tend to be fantasy in actuality, so fantasy it is for this genre.
Isekai is often used as a mechanism to make the MC OP, but not usually because they have more advanced knowledge. While I enjoy a good OP MC isekai, I'd much rather see an intelligent MC who uses their knowledge of physics, for example, to get ahead. In the vein of Wise Man's Grandchild in how his comprehension of atomic theory allows him to more efficiently and effectively utilize magic. Or Log Horizon as far as the crafting of steam engines goes. Or in the vein of the Binding Words series (Daniel Schinhofen) where the MC is a crafter who (slowly) brings indoor plumbing to the masses. Still OP, but not handed out like Halloween candy.
On that note; crafting. I'm less interested in, say, blacksmithing or runecrafting than I am in the MC using their knowledge of earth to craft things. Combining the magic system often employed in isekais with science tends to be very interesting to me.
In that sense, the MC can be OP, but not godlike. Developing a party and allies is critical if you don't want to live as a hermit. While kindness can be an effective way to attract and retain quality allies, these isekai worlds are often brutal. This needs to be hammered into the MC hard and early. Obviously there needs to be some adjustment process when killing sentient beings, maybe even goblins, but a lot of people hunt their own food in our world. Let's just go ahead and stop being squeamish about killing a non-sentient animal for food, yeah (humanely!)?
I tend to lean toward anti-hero. Not exactly grimdark, but isekai, almost by definition, means no ties connected to the MC. They aren't indoctrinated into a religion's or country's definition of good and evil or morality (as if the concepts weren't entirely subjective anyway). The MC should do what needs to be done to protect them and theirs. Reactions in our world are tempered in a lot of ways. Civilization can be said to be beneficial, but it also tames the tamable. In an isekai world, this taming factor would be much less pronounced. If an enemy attacks you, you run far enough they can't find you or you take away their ability to hurt you. Grabbing a tiger by the tail is dangerous for a reason. I'm not saying they should be an asshole. I'm saying they shouldn't start fights; they should end them. Decisively if possible and if necessary. This is... not easy to do without the story becoming a teenage masturbatory power fantasy, so I understand why it's not more common.
One overarching question on the genre, though; why would anyone want to go back? It's just not something I can understand, viscerally.
While I do like me some smut-sekai, I don't care to read (smut/erotica) without relationship building involved, so if it's just written porn, I'd just as soon pass. Reading is as close to isekai, metaphorically speaking, as I'm likely to get, and I tend to immerse myself pretty deeply into whatever I read. I can't do casual sex in real life, and it's not always easy to stay immersed when it's in writing (for me). Harem is fine, but the people must be fully fleshed out or what's the point? It's a common mistake to overfill the harem and underfill the people in the harem.
I'm certainly not saying all of these ideas have to be in the same story, but they tend to be what I enjoy.
