After playing through the 0.54 windows version, here's a list of ideas that came to my mind. I've broken this into three categories, bugs, gameplay experience improvements, and other ideas. Given these were just thoughts, take them as is and use them if need be.
Bugs:
For the bugs, I encountered two. These could be handled already, but will list them here regardless.
-When entering the tentacle area from the exit (after you have cleared through that area and reached the outside final portion of this demo), you end up on the other side of the sewer. Perhaps this was intentional to make it faster for the player to reach that side without going through the tentacle portion again.
-Upon reaching a game over by the futa zombie school girl, the autoplay isn't properly timed. It ends sooner than it should and skips the part where Syahata is changed over to a zombie. It plays fine in the gallery mode.
-Should be kind of obvious, but enemies shouldn't respawn upon re-entering an area. Maybe this is intentional? But I feel like the enemies shouldn't be respawning.
Possible Gameplay Improvements:
These are some thoughts I had for what could potentially improve the overall gameplay of the game based on the 0.54 demo. Overall the gameplay did feel fine, but these are just some suggestions.
Additional Challenges
The game is honestly quite easy to play. In some cases it feels challenging just for Syahata to die. Also the only time you see the explicit scenes is when you are grappled and it takes some time just for the enemies to finish. This can be annoying for the player as it hinders them from moving or it requires some time just to get to what they want. And the player can easily avoid getting enemies to finish in Syahata through masturbation. Thusly, here's an idea that might help balance some of the game.
Lust meter/Corruption marks
As what I can tell, Syahata's dialogue and give up pose changes depending on how much sperm she is containing. Perhaps its a combination of the amount of times she's had sex, but regardless this idea is to add an additional affect onto Syahata. The lust meter will essentially do a similar thing as it is already, but add some additional challenges for the player.
The higher the lust meter the harder it is to break free. Of course don't make it actually harder to mash out, but make the finish meter fill up faster. This way it is faster and doesn't require the player to mash nearly as hard for as long. So the escape meter will be the same, but make the finish meter increase at a faster rate given the current state of Syahata's lust. Perhaps it could be incremental as well. Such as if her lust increases the bar fills slightly faster based on that amount despite not exactly seeing the physical changes to her body/dialogue.
Once Syahata has reached the max lust state, she should instantly give in and have the enemy swiftly finish the process of ejaculating than waiting for the whole meter. After her lust state has expired, she will go back to her normal non-lust state. She should lose the lust state once the enemy has finished in her. Thus, with the constant increase of Syahata's lust, will end up forcing her into being finished in and will be much faster than having to wait the full time if it were to happen. It also adds some challenge because there can be some additional affects to Syahata for when an enemy has finished in her.
For how the lust meter can increase, perhaps we can consider the lust meter with a different name given it will reset when the enemy has finished in her. The name it could be called is corruption marks. A corruption mark could be added each time Syahata takes damage or potentially through some airborn smell or an additional move that the player would have to dodge otherwise further her corruption. Perhaps the mark could form parts of a heart on her abdomen. The completed heart means full corruption and max lust.
Pregnancy and Sperm Additions and Changes:
Here are some ideas that involve pregnancy and the sperm count in Syahata. Firstly, I think there could be some dialogue for when Syahata is giving birth to indicate her thoughts like when enemies have attached to her. There should also be additional birthing poses than the one with her on all fours. Perhaps a missionary type pose or a standing up pose. This will provide more variety for the player and may be experienced a lot more with corruption causing enemies to ejaculate in Syahata.
There should be a model change for Syahata when her pregnancy comes further along. Where it is represented in the pause menu, it isn't shown on the gameplay model other than the birthing animation. Perhaps she can also slow down while pregnant which could add some challenges for the player.
For some of the sperm changes, following the concept above with the corruption marks, I think it can remain as how I am picturing the current lust is with the amount of sperm within Syahata. So, with the corruption concept, when Syahata has had sex and it resets her lust, it only would reset her lust down the amount of sperm currently contained in Syahata. Thusly, the amount of sperm would be Syahata's default corruption.
Now the problem with this is that it would be possible to max her corruption/lust and always be auto give-in to every enemy. Hence why it would be wise to plan a way to remove the sperm contained within Syahata. Perhaps one way would be to make additional pregnancies for each of the type of enemies. Such as a humanoid monster with the zombies, rats, puppies and more. This would give a lot more variety for the player to have to deal with. Additionally, can make the sperm count be a threshold instead of being some sort of random chance. So the amount of sperm to contain would have a max ceiling when reached would cause Syahata to become naturally pregnant than pregnant through enemy eggs.
Other Suggestions:
Lastly, these are some other suggestions I have thought of that could have some potential but not as important as the others above.
There could be additional game over art depending on Syahata's lust. Such as having max will cause her to be excited for her impending doom when compared to when it isn't.
Perhaps change the art in the game over for the futa zombie school girl where there's an x-ray on Syahata to showcase the other girl is currently in her while Syahata is in the girl below her, but only on those sets of animations with Syahata as a zombie
Some additional thoughts around the concept of corruption.
A special type of corruption meter (possibly not the same as lust/corruption mentioned above) changes Syahata's appearance such as her slowly changing over to a zombie, a dog, fly girl, or even some tentacle entity
This form of corruption can be healed in a way that I do not know of, but wouldn't be a permanent change for Syahata.
Max corruption leads to an automatic game over. This would go hand in hand with the appearance change since the transformation is thusly complete.
Corruption is what allows Syahata to naturally become pregnant with the entities that have sex with her. So perhaps the flies and tentacles wouldn't need to plant eggs in her as she already contains the eggs for them. This could go hand-in-hand with the appearance change where based on where her corruption form is taking Syahata is what Syahata could become compatible with.
These were just a set of ideas that came to mind when playing through the 0.54 demo. Keep at it.