A story where best girl wins
Here's also some less ambitious ideas:
Demon lord gets isekai'd into the real world as a weak loser otaku with no power at all. He then sets out to regain as much of his strength and influence as he can, though he mostly appears like a chuunibyo to other people and doesn't make all that much progress except for getting /fit/. He slowly learns to live a normal life and respect other people, but still strives for as much greatness as he can.
Guy and girl, unhappy with their lives switch bodies due to some wish/magic. Unable to go back, they have to live that way for now and learn the good and bad (see: not just the bad) sides of being the opposite gender. The new change prompts them to try harder in life in /improve/ themselves, slowly get accustomed to their new bodies. Eventually they find out how they switched and know how to switch back, but resolve not to since they're happier that way, and end up living wholesome lives as gentle ikemen and genki tomboy. They won't fall in love with each other.
Mahou shoujo where MC becomes the first "magical boy" in a world where magical girls have existed for as long as people can recall. It's basically a Stardriver clone, very flashy the MC is similar as well, but it's more about MC trying his best to get into terms with his new situation and improve at magical stuff and stack up to people's expectations and defeating villains and gets his shit pushed in more often.
Comedy about a young dad who wants a cute daughter. When he has children with his waifu, he gets a son 3 times in row, all of whom are weird in some way (e.g. one doesn't speak or express anything, one crossdresses secretly, one is almost impossible to see because he always disappears somewhere etc). They try one more time and he finally gets a daughter, but she ends up being a huge tomboy and the least girly of them all. Story follows their shenanigans. The siblings don't like to interact much, but when they do, they each have some kind of special skill that lets them get through things.
Shitty romcom about a girl who's amazingly beautiful and graceful and seems like the queen of the world no matter what she does, but is actually a complete idiot and sucks at pretty much literally everything. She's aware of it and tries her best to do better, but to no avail.
Story that focuses on exploring the fundamentals of magic and going into high detail about how it works. Magic in the world has been "degenerated" in a way that people only know how to cast spells by using runes and chants. MC starts exploring how magic works and finds out that what's known as "ancient magic" requires fundamental knowledge of magic, and lets you manipulate magic freely and quickly. Runes and chants have an advantage that they make it easier to cast complex specific spells, but they're completely rigid, can't be altered from their design, and can be nullified by an opponent who knows the spell.
I have a more detailed outline for a story using this idea, and I'd like to try turning it into a manga someday. Long story short it's an isekai where the MC learns magic differently due to being brought from another world without magic, and tries to find a way to return home. He's not more powerful than others nor gain any power after being transported, but can do certain things that others can't due to a different way of using magic.
Not a story as much as a subject that can be made a story out of or put into a story. There's a weapon called "dagger of jealousy". It can effortlessly cut through anything, but that's not what really makes it dangerous. What makes it dangerous is that the longer you stay physically separated form it, the more it starts to mentally affect other people around it, causing them to search for the dagger and use it to kill it's true owner.
There's 2 versions to the idea, first is that the owner can't actually be killed with the dagger because by some happenstance it always ends up killing the wielder (they trip and fall onto it or accidentally cut themselves when you try to stop them or something similar). Second is that once the dagger is wielded by a non-owner, they will not return to their original mental state unless stabbed with the dagger, and they will not remember wielding the dagger once stabbed.