Hikigaya Sensei
A little similar to the Hiratsuka Sensei with a smaller age gap idea.
Hikigaya is in his early to mid 20's and has finished his education and just starting his first teaching job at Sobu High School.
This is set at the same time as the original Oregairu series, just that Hachiman is a different age.
Could maybe increase Komachi's age as well so that she is the same age as the rest of the cast like Yukinoshita and Yuigahama or the year below in the same class as Isshiki.
Hiratsuka Sensei will be Hikigaya's senior at the work place and she's tasked with helping him out and getting used to the place a bit better.
Hikigaya isn't quite the edgy teen he used to be and has matured a bit.
Hiratsuka Sensei makes him get involved with the Service Club as well to help her and help him develop and deal with slightly troublesome students (Yukino).
Can have Yuigahama and Komachi join the Service Club as well
Could make it that Hikigaya still saves Yui's dog the year before but when Hikigaya started teaching her class a year later was the first time she's seen him since and wants to make him cookies like in the original. This leads to her joining.
Of course Hikigaya isn't going to be too hands on with the club as he has other duties as a teacher and wants the students to solve the problems themselves.
Hikigaya and Hiratsuka get along well with each other as they find they have some similar interests and have their banters like they do in the original (though he can't really make the age jokes now).
The other teachers and some students tease them for being close, including Komachi who tries to ship her brother with Hiratsuka.
Hikigaya and Hiratsuka do things like go to ramen stands for dinner after work or go get drinks after work finishes for the weekend.
It would be interesting to see how Hachiman being a teacher instead of a student will change how the story plays out as well as the different relations and interactions between characters.
Some examples I can think of are:
Yukino might show more respect and be less insulting when first meeting Hachiman but if she isn't then he might be able to put her in her place a little.
The Service Club will operate a bit differently with Hachiman being replaced with Komachi.
Hachiman and Hayama won't really have their sort of rivalry since they are different ages and Hachiman might even become someone that could give him some advice instead.
Hachiman might help Saki in this situation as well like sorting her family situation and giving her advice on her studies. This could lead to him becoming a likeable teacher to her.
Yuigahama would likely not have a crush on Hikigaya since he's a teacher but you never know.
There's a bigger age difference between Hachiman and Komachi with Hachiman being a bit older, so their relationship will likely be a bit different to the original but it would be interesting to see them get along as teacher and student as well as brother and sister.
Hachiman and Haruno could have an interesting dynamic since Hachiman will be the older one now and will have more of a defence to any of her flirty teasing.
Haruno might do it to try get a reaction out of Hiratsuka since she sees how close he is to her former teacher.
Though she might be testing him to see how he is and if she 'approves' of him.
She'd likely try to get involved for something relating to her sister as well.
Another interesting thing to see could be Hikigaya meeting Yui & Yukino's mothers at a parent evening.
So yeah, eventually you would have Hikigaya and Hiratsuka become increasingly flirty with each other and start going on outings with each other more and more until they eventually become a couple.
Lemon chapter between the two could follow.
