I know I had this mentality before, not with your stuff but with some hentai that have harems but only focus on one girl anyway. I always think, 'why do they keep going back to this girl? the other ones are way hotter and we haven't seen them yet!' but aria doesn't really have that problem so, I don't know why they'd think that here.
I would like to see the teachers in UMCH though, but that's just b/c I like glasses girls and I'm not sure if Roise and Amber are confirmed for sex scenes yet.
For ideas on out of character sex scenes you could have it be something like a recording you find when going around. Like if Lynn has no reason to have sex with Jeo, you could still have a scene with Lynn by having Phia find a recording of Lynn fucking some guy, after she completes a raid or maybe just buys it off a shop keeper.
This could also work with flashbacks to different events or even a character just fantasizing about it. Though flashbacks could only be used once in a while to not get overused and most characters in your stuff wouldn't be fantasizing about stuff like that. Except maybe Kyle, Amber or possibly Chris.