The premise is simple: a simple game like Poker, or maybe Blackjack, or some other simple-to-implement / simple-to-play game where you can make bets. The in-game context would be you playing against other people who work at Rack Laboratories after the doors are locked. You pick a few fetishes, you assign which get employed with winning and those with losing (and maybe those involved with just playing; maybe the whole game is played with each participant under the influence of a vibrator! Or something) you both hop in your racks across from each other, and play! You get to enjoy watching the results.

Fun things to consider: Maybe the game is played PoV, giving you an intimate look at whatever Winning or Loosing looks like for your junk, while you click on in-game interactables (with your mind! Wom-wom-wom). Maybe the fetish involved will distract you or your opponent in some way -- that is, maybe the mouse becomes *really* jittery when you are under the influence of post-orgasm rubbing, or maybe you get to hear your opponent say 'Damnit I didn't mean to click that!' because they just climaxed mid-turn, adding to the strategy. Perhaps there could be achievements associated with it.

Similar to tentacles, urethral sounding, cbt, and oviposition, often but not limited to, a penis. The tentacles or whatever jam in through the urethra and you can see them distend through the urethral canal all the way through in to the ballsack as they play/squeeze/eat the testis.
This fetish often interplays with internal views of anatomy and whatnot, everything from cross-sections inside the penis, to x-ray views inside the ballsack.
Could be fun considering it's also a cross-section of a lot of fetishes.
This fetish also uses maggots or insects, which I am not a fan.

I have some advise the game seems to auto save everything (all the time) you might wana change it to have a manual save for insurance, and to make it less resource intensive it would help to change the unused particles.

that injects cum inside the characters ass or vagina doing infinite creampie

Basically, you have a system whereby you can put out ads for the cost of money. The more preferences you specify, the more it costs; maybe the first three cost some flat rate, but every one past that gradually costs more and more per preference. You' could spend more on the add to try to get more volunteers, and they'd be paid as well. But ... it would give the players another avenue to spend currency, give them subjects that enjoy the same things they do with a sense of determinism / less of a sense of RNG, and be contextually relevant to the game's world.

It does feel good lol.

Tongue penetration and tail penetration would be really good

much like how in real life there's people that like to finish in a specific manner, there could be the same thing for characters within Rack Furry Science.
some Taste ideas:

-Cum Inside: the name says it all. the character likes it most when their partner is cumming inside them at the moment of their orgasm. they can get offended if you cum outside or if you slip out and don't try to put it back in while you're still cumming.

-Cum Outside: the oposite of the Inside. the character dislikes being came inside and it won't hesitate to let you know about it.

-Stay Inside: The character does not care if you cum or not...but it does want you to stay inside as it is orgasming, and maybe even when not necesarily orgasming at all, and this is just to enjoy the feeling of closeness (maybe even a cuddle)

-Stay Outside: The character does not want to be penetrated at any given moment, and doing so could actually count as rape regarding the character being penetrated. the character will instead greatly appreciate it if you limited yourself to grinding

-Internal Grinding: the character likes it most if you're grinding inside it during and after orgasm.

-Internal Pressing: the character desires for you to keep pushing inside, thus pressing their vagina with your phallus. it differentiates from Stay inside on the sense that it requires you to constantly push forward and not letting go of the movement key.

-Deep Cumming: this one is similar to the internal pressing, except that its given window of oportunity is given while you're orgasming. if you keep pressing (specially effective with characters whose penis is long enought to reach the entrance of the cervix) and you get your phallus properly alligned, the character will be quite pleased and some extra money can even be expected to come in later on.

a lot of people are really into their character having special magic powers, and a lot of dragon furries like having that charizard tail, so i wanna suggest having the option to apply certain special particle effects to your characters paws, wings, eyes, horns, and the tip of their tail (and maybe around their genitals?) in the character creator

stuff like flames, electric sparks, dripping liquid, bubbles, leaves, smoke, and just generic sparkly particles, all color customizable