Things like eyepatches, customizable collars, earrings and piercings, bracelets, necklaces etc.

an option for a genital "slit" that the penis will slide out of when erect, as often seen on reptile/dragon, bird, and sea-creature characters

Can be penetrated like a vagina

it'd probably be hard to implement, but i think it'd a cool touch to add some "physics" options, where if the penis is particularly small, it can still be penetrated while erect, or if the penis is too big it won't fully retract back into the slit while flaccid

There should be a category for humiliating the subject while forcing them to cum. as its own specimen I mean, since there is going to be mind control you hypnotize them afterward to prevent any ill effects of having been essentialy broken during the session (wether they remember what happens or not)  

Lots of Bras, Panties, and Swimsuits to be worn

I would like to have the ability of letting a tentacle makes its way through the gastrointestinal track, starting from either end, and that would be pretty cool if this game has that feature.

Inserting a characters muzzle into another's pussy/ass

i think you should add in-game cheats like in rack 1 and allow mod support

Diphallism is a natural phenomenon in the world on snakes and sharks also occasionally humans so I think it would be a good option if it wouldn't be too much trouble. And the ability for 2 different types might help with creativity or creation of currently existing fursonas