Something silly that I was thinking about would be a girl who, for some reason, has to crossdress and join an all-boys school - something extremely overdone and cliche, I know. However, the one guy she falls for/ends up with turns out to be another crossdressing girl.
Preferably the story would switch between the points of views between the two girls, with them getting closer to "that one cute guy" but feeling reserved about getting into a relationship because they're trying to hide their genders and don't think the other would be interested in men. They'd find out of course (though hopefully not for too long), either at the same time or separately, and try to deny their feelings at first but give in eventually.
One reason I'd find this type of story interesting is that most if not all of the supporting cast would be male, which is definitely a rarity for yuri in general. Like, it'd be cool for both the female leads to have close male friends that they don't end up with, even if the girls are shown to be interested in guys to some degree - bisexual yuri is pretty cool. And if the guys were to find out that they were crossdressing, they'd be more or less cool about it. There'd be some teasing/flirting/awkwardness, but in the end they'd be friends regardless and later on even be supportive of the girls' relationship. Also at least one yaoi-sidecouple, though the main focus would always be on the two girls.
Genre-wise, it'd ideally be a comedy-drama with a focus on the character's thoughts and feelings and such. This would most definitely be a high-school drama type which I know a lot of people are tired of, but that's the sort of thing I like so eh. Come to think of it, depending on the reasons for why they have to crossdress and such it could have a deeper plot, like some sort of conspiracy, mystery, or other such complication, but I'm probably not creative enough to do that sort of thing well.