Great game with lots of potential. I love how clean it is so far. No shota no urine/feces and stuff like that. The concept is also pretty good, they are trying to drain the demon lord.
That being said here are a few ideas:
Enemies can be given grab moves as their attack instead of generic slash or bash attacks and if protag is knocked down they can use the sex attacks.
Keep enemies varied. I like how there are several types of enemies. Theres a human warrior (please add more humans trying to take oppurtunity to drain the lord for themselves... an amazon woman, a wizard, an archer etc)
Add more monster girls like Minataur Harpy Centaur Lamia etc
Add more human like enemies that belong to other races like the orc. You can use elf, goblin, dwarf and giants as different races
I know this will be complex but maybe think about some enemies double teaming on the lord?
All in all a great game.