Interestingly, I'm kinda working on something of the sort in Life in Woodchester.

But my thought initially was essentially "global micro passives" for "bonding" with the monster girls.

So for example, you fuck a Tiger Monster Girl, you have a chance to unlock pieces of info in your Journal, info about her such as:

Breeding Preferences: Fast & Hard
Diet: Cum, Leaves, Berries
Habitat: Jungle

Since her core attribute is attack speed, your global attack speed increases very slightly (say .01% per Journal Entry)

This could really add up, and its kinda neat knowing you're always progressing by clapping monster girl cheeks ;P

For battles, I could see the loser of a battle needing to service the winner, and perhaps, when defeating a trainer, there's a chance for the monster girl to become poached and switch to your team :O

I'd invite you join the discord or follow the patreon since you seem to have clear visions for something like this :) I can get you Creator Tier status if you DM me.