I want to make some suggestions. I don't remember what I've already said and what I haven't, so I'll list everything that came to my mind.
For men, you need to remove your pants or significantly damage them in front. It looks strange when their penis grows out of their pants during the mating animation. If you tear their pants, you will need to draw them an erection and a masturbation animation, like slime's.
Women should also tear their clothes. In particular, the shirt, in the chest area. You don't have to touch the skirt. You can add underpants to the foot of one leg. Smudges of semen on the inner side of the thighs and /or dripping of sperm, as in Syahata after mating. Also add masturbation animation to them.
Make several different models of female characters. With different hairstyles, hair color, torn clothes in different ways, possibly different clothes.
Add zombie teachers to the school. Add civilian, military, police and other zombies.
As I understood it, the virus in the game causes strong sexual arousal and mental confusion. Which makes infected people constantly look for a partner to satisfy. But they don't pay any attention to each other at all. Make an animation of mating between infected men and women. When Syahata passes by, they stop and start attacking Syahata.
Make an attack by different opponents at the same time.
Add fast zombies in athletic form.
Add the alpha position of the Syahat at high debauchery.
Add the count of the eggs that Syahata was pregnant with. Now it shows how many eggs and what kind of creatures are inside at a given, specific moment. You need to make a separate counter for how many eggs and what kind of creatures were inside at all. It is possible to make the same separate invoice for partners and/or their sperm.
Where did the teacher get the gun? Replace it with a can of some kind of chemicals. Sprays a cloud of gas at a short distance, endless ammunition. It is very effective against insects (deadly), useless against slime, incapacitates other opponents for a short time (except the boss). Thanks to him, you can run past without a fight or gain an advantage.
Add a unique opponent to the school, a mini boss policeman. He has slightly more health than an ordinary opponent, he moves faster. Aggressive in mating. Having sex with him takes away HP. After defeating him, Syahata will receive a gun.
It is possible to replace the pistol with a revolver, and make it more powerful. More damage, more recoil, after the shot you need to adjust the sight.
Add in-game achievements, possibly with references to movies, games, songs. For example: "Exterminator" - kill 10 insects with a can of gas. "Master of Kung Fu" - kill 20 opponents with a stick. "Syahata Rambo" - kill 10 opponents with an assault rifle in 5 seconds.
There are also similar achievements for mating. For example: The "Queen of Flies" is to give birth to 20 fly larvae. "Sasha Grey" - to get sperm from 20 male opponents. "In the name of tolerance" - to get sperm from 20 female opponents. "Zoophilia" - to get sperm from 20 dogs or rats. "Nymphomaniac" - to mate with 100 opponents.
And achievements for losing: "Incubator of fucking larvae" - to lose with a fly. "Gangbang Center" - to lose with a man. An "all-female party" is to lose with a woman. "Rat Queen" - to lose with a rat. "I really, really, really love dogs" - to lose with a dog. "Professional sports" is to lose to the boss of the gym.