I really like the game and it`s development so far. There are some ideas nevertheless:
1) The size/muscle drain from the Lost Naga could be implemented to other charakters but not by swallowing like the naga does:
It would be nice to see a demon (Amagnathon for example) which is rather small (compared to the others) but he gains muscle, height, cock size, ball size by different sex choices. If the MC tops him he will loose muscle which is transferred to the enemy. If the MC bottoms for him he will loose cock length in the process. 69 will drain his balls. While getting a blowjob from the opponent the MC will loose height while cumming. All of the aspects above will only apply if the MC cums first (arousel too high). The MC will gain his size back and even drain the enemie if he makes his opponent cum first.
2) Another idea considers the recruiting of the lost creatures when the corruption is high enough:
When you fight Apothus there is a scene where all Lost merge with him to give him size and strength. It would be nice to gain such a special ability when the MC is highly corrupted but low in strength and feasts of his followers.
3) Lukka could craft a demonic buttplug out of the demonic essence which absorps the corruption of the holder the longer it is inserted and the more demonic shards are used. The MC could use it to fuck corrupted beings without getting himself corrupted (purity route) or it could be used to strengthen the Lost companions like the minotaur or the recruited lost creatures.
4) The last idea is about the creatures of the Nynx, which go into the balls of the MC in the castle:
If the MC gathers enough creatures in his balls his cum will have special abilitys on the enemies but only if his cum hits them. At a low rate it will increase their overall size. A medium amount of creatures enables the MC to be in charge of the growth (height, muscle, dick length, ...). A Large amount of the Nynx creatures will give him the ability to drain overall size from his enemies and make himself bigger.
I appreciate your work very much and hope that these ideas will be helpful for future projects. (I appologize for any bad spelling but I´m not a native english speaker;)