Tunneled buttplugs that allow penetration, free flow of fluids and  keeps holes open.

Just writing and text on the body. Could be for humiliation, or just tallying up how many times the subject has been used/orgasms.

I haven't seen this anywhere, and having had the idea I don't want to risk forgetting it. So I'm talking stuff like this https://e621.net/post/show/897540/2016-anthro-anus-belly_scales-bent_over-big_butt-b But maybe you could have the option to crank it up or down, and change the color. Maybe there could be a species, or it is just a rare thing that anyone could have. You could take the dna, enabling you to put it in other subjects. Perhaps their cum/fem cum could glow as well. Making the colors match could be optional, allowing the player to have any combination of them. As an offshoot, perhaps it could affect lactation. Affecting color and possibly flavor for both.

Not just genitalia portals, but large ones so you can have for example half a character (butt) coming thugh the wall while he other half (torso) is on the same side

Not sure if this has already been suggested.

If a test subject has a huge cock, it'd be hot to be able to fuck it. Put your dick in their urethra and fuck it.

Not only that, but if their cock is smaller than yours, you can envelop their cock with yours, kinda like a fleshlight.

Since it looks like Rack is getting an update to fluid physics, why not add more fluids to the game?

The hymen is a membrane found at the outer opening of the vagina, and is typically torn by careless lovers, emitting mild amounts of blood.