First, I really enjoy the game. Glad someone's making stuff like this.
Now I'm sure you've got (and get) more than enough of these but I thought I'd throw them out there anyway. Who knows, maybe you'll take a shine to one.
1. Most of the enemies currently in the game are focused on busting their nut in you, but what about an enemy who wants to make you get off instead? Her goal in battle is NOT go cum, but to make you do so instead. She's taken a shine to you and wants you to finish first. Of course the quickest way to a man's balls is through his prostate, so she'll be trying to do this like every other enemy (though now and then she could try a special move for a tit/blow or handjob). Once she gets you off, she's satisfied with that and will pull out and leave, blue balls and all. She's doing this to train you, to prove she's the one in control while subtly taming and conditioning you (through pleasure) to only be pleased by her, as well as build up for the day when you begging her and only her to be yours. Her trait will make you last a longer against other monsters but become weaker to her. This can be useful, but if you go to far you'll find she's the only one who can pleasure you anymore and at that point she's finished, she'll finish, and you're finished. Of course if you take control and make her pop first in combat, she'll have to start all over, but it's no easy task as it takes a very long time to get her going if she doesn't want to.
2. My other idea is a regular woman who's become enamored with all the other monster girls running around. She loves watching them and and wants to be like them, something about the way they take a man and plow him like a woman fascinates her. She doesn't have a dong, so instead she walks around with a strapon all the time, and her dream is to find some cute boy and follow in the monster's footsteps. She's not particularly strong though, so if someone is too strong she'll more than likely try to trick them, offering them a drink or something to eat. Many a man's accepted her offer only to wake up with a leash around his neck, cuffs on his hands, and a monster girl getting far too friendly with his backside while she's holding the leash and giggling at the scene. Get tricked too many times, and she may make that leash permanent. If she meets someone she can beat in combat, in addition to her dildo having its way with them she'll also give them a little gift to make finding a friendly monster easy. She'll lock a ring gag into your mouth or slather your butt with pheromones that will cause monsters to bypass a fight and go straight to their bad end risking scenes.
3. I also like the character Trudy, and I was thinking that maybe he could have a bad end of his own. He seems pretty arrogant, and if you fail to avoid his trap and take him up the backdoor in his fight, what if instead of joining you he forces you to join him. He decides that you are obviously not leader material, but your ability to please others (and himself) could be useful. If you're not strong enough to resist, he'll make you his pet, dominating you when he feels like it and throwing you to the wolves in order to escape any fight he can't win. Your adventure is over and you're nothing more than a rich boys' boy toy.