I have been following Annue's Tumblr for a long time and I was so happy to see they followed through with that GIF that started this whole game idea. Also, Annue's art is definitely perfect for this game and so cute and adorable overall.
I'll jump on the idea suggestions bandwagon:
1) Give the player some more involved objectives. The "fuck X kinds of girl" idea does it's job for this early version, but it doesn't qualify very well for giving the player some reason to keep playing the game after the titillation effects wear off. I suggest you could make a small gameplay cycle in which the player starts off with very few lockers, and that fucking the girls awards the player with a few resources (cum? milk? pussy juice? the possibilities are vast!) that can be used as a currency to buy upgrades, such as more lockers, better tentacles in lockers for more sex positions and sexual activities and whatnot, etc.
2) Give the player some conflict or something to be wary over. The girls do show reactions to seeing lewdness and escape. Maybe they could call for authorities if they successfully escape, so the player now has to strategize when and where to catch and release girls, which can be a little and fun challenge to spice the game up.
3) Since the girls are clearly in a school setting, you could leverage on this to add more mechanics. For instance, maybe there could be doors to classrooms so girls come in and out from classes at given times, and the kind of class could dictate the kind of girl that comes out of it. Maybe if a girl is coming to attend a class and you happen to catch her before she reaches the door, you can make her miss her class if you don't make her cum fast enough, which can have repercussions akin to what I just suggested above. Or even the tentacle monster's cum could have stimulant effects that makes the girls do better in school, who knows. Also, you could add hot teachers, because hot teachers are a must in a hentai game set in a school.
4) Add in "special girls" just so they are unique characters that can stand out from the generic ones. These special girls can keep appearing again, maybe even sometimes looking for a locker they have been caught in before because they want some more of tentacling in their intimate parts. Maybe each one of these special girls have their own unique tastes in sex so the player can build some kind of relationship with them if the player manages to fuck them good.