You can achieve all those interactions even through a primitive VN where you must pick from 60 choices which each match a different character in order to score high. Very similar to dating games, it's sad how porn games have actually gotten worse over the decades.

https://www.youtube.com/watch?v=uelGUXRqi1M

You can have a unique UI per character and have something like a cursor vs cursor battle as you try to hack the girl's UI to gain control over the new options and vice versa.

The physical biology should be just as important as the neurology since each pony does not have the same body and cannot take the same exact cock and thus being able to customize your cock would be interesting. Something like each species having different properties.
Would be even better if it had physics code and automatically interacted in a meaningful way instead of arbitrary things. The stats should be actual stats and not leaderboards for how many cocks you sucked while it doesn't help with anything... at best with unlocking some unrelating thing like story progression if you became a faggot and sucked over one hundred marecocks.

Anyway the key ingredient is indeed interaction both in bed and outside bed. Outside being how the character behaves around you depending how you fucked her. Will she turn yandere, prepare your morning coffee?
What is the world outside the bed? 3D Minecraft, hack n slash RPG? Is your GF your healing priest bitch and will she pretend she couldn't heal that spike damage if you didn't fuck her well? Will she overvote and over roll for that legendary item and if you die refuse to revive you to get your endgame chest? will she not take it personally and just do her fucking job like she's supposed to regardless of how well you fucked her?

Will you be a werewolf and play both sides? A rapist and a lover? as you contradict and ruin your own life in a Dr Jekyll and Mr Hyde kinda way?
Will the game be about 5 lesbians & 1 bisexual ponies and you must murder the other 5 so they stop stealing your 1 true mare away from you? like a reverse gender yandere?