Most courtroom dramas have the judge and attorneys written as men, but don’t have to be cast that way. There are some that are period pieces, and couldn’t use women, but most of them could.
The Bad Seed was more shocking in its time with a little girl as the title character, but I don’t see why a boy couldn’t be cast as the Bad Seed. Especially if you wrote it for the 21st century, so for the lines about the child wearing dresses could be replaced with lines about not wearing jeans and sneakers all the time. An 8-year-old boy in a button down shirt, dress shoes and slacks when he has the choice to be in jeans, a T-shirt and sneakers, has the same impact now, as a little girl who insisted on wearing dresses all the time did when the play came out. You could also go either way with the parent who is concerned about having passed on the bad seed, although that probably takes a little more rewriting. But the opposite parent being in the military and sometimes having several-day absences could work with a mother now.
You could try Same Time, Next Year with two men or two women. If you try it with two men, you need to come up with a device for the year the woman is pregnant-- maybe one of them just had a vasectomy.
Children of a Lesser god was a vehicle for the Deaf actress Phyllis Frelich, which is why the central couple is a Deaf woman and a hearing man. You could probably cast it the other way. Right now, casting them as a gay couple still adds too much baggage, and distracts from the central Deaf-hearing conflict, but in ten more years, that might not be the case.
Casting The Odd Couple as platonic, opposite-sex roommates might be interesting, except that in order to run counter to expectations, and make it funnier, you’d have to cast the Oscar character as the woman, and having a platonic straight couple might reinforce the assumption that the play works so hard to dispel-- that Felix is gay. Of course, it might not matter anymore. Not that you’d want to make him overtly gay, just that it might not be a problem anymore if some people assumed that.
Actually, it might be fun if someone (if Neil Simon is getting too old, but would authorize someone else) would write a version where possibly old college friends, male and female, one out for a long time, one just out and just divorced, decided to room together, and had a fussy/messy conflict. In that case, you actually could play a fussy lesbian, and a messy gay man for laughs.