What kind of new monster girl race do you want?
I think there's a lot of places the devs could steer this.
I expect it will get more complex in the future, as it's basically trivial on the hardest settings right now once you know how things work.
The dev could make a whole science around animal abuse to create the furries, and take them out of the base game, forcing the player to perform bestiality or weird shit like forceful egg insertions on pregnant women to produce harpies, and the more regular methods for centaurs, tiger/lion/pump/cat girls, dog girls, goats, whatever. Egg insertions for giant ant/spider/various-bug girls too. You might need loyal girls to do the work, unless you can just get giant bugs somewhere to lock them up with. Or maybe a stall where some poor slave can be given a somewhat normal job jerking them off for DNA if you'd rather skip the normal unnatural manner to create them. Could be fun. Plus centaurs.
A food simulation could be interesting, growing mushrooms underground and fields at ground level. Animal husbandry that allows you to gather cloth to have sewing slaves create a library of clothes so you can outfit your slaves if you want. Dyes etc. Mincing the animals could create both hides and flesh.
Have the lizards require regular large amounts of meat to make them harder to keep around. If they get hungry they should start taking damage and after half their life is off become ravenous, eating any girls you try to have them mate with. You could drop dead slaves into their pins as a different way of recycling them. Or have the minions mincing the girls produce a slave meat by-product that the lizards would need, probably 2-3 slaves worth, making it reasonable if you want the other by-products, but cheaper to just feed them a few unwanted kids or small-races or one large one. Or just a whole fucking load of babies, because hey, those aren't filling anyone up.
Torture devices and torture shows. Slave fighting for both male and female slaves. Introduce bloodlust that killing mollifies, so the player has to toss goblins into the arena for the werewolves to just absolutely shred to shit until their bloodlust goes down enough to have them mate for a bit, otherwise risking them tearing through the girls instead ( generating those corpses you need for the salamanders ).
You could have a necro-room in the brothel for those real freaks ( how many bangs does a corpse has before it lets off a roomful of miasma? ), and have the local town send investigators you'll need to ensure don't see things through some kind of guarding mechanics. Maybe have the town send out medieval CPS to stab you over all those kids in your brothel. Really, what the hell, man? Make them royal inspectors, and have them get upset over seeing animal girls since you'll have to create them with evil science and trading monsters is super illegal, right? Perhaps just make it a mechanic where they sneak from room to room till you click them with room guards you place alerting you, and then you have to either bribe them ( depending on how much they saw ) or kill them, which will result in more/harder inspectors heading out.
A scene of the PC getting hanged for his crimes after the palace sends in the army since you keep killing royally sealed inspectors could be nice.
Some nice rooms could be good, allowing your girls to rest outside of just the current slave pen. Some fancier ones for the estate could be nice, allowing you to train your daughters in histories and magics, the fine arts, music and dance, and generally just get them really well rounded before you butcher half and send the rest into the arena to be torn apart by starving orcs.
Maybe a mechanic where you can pay to have a match-maker find them good homes ( requiring certain levels of education, and a larger fee with the match-maker for better placement ( +3x dowry if placed as a wife instead of mistress ) ). They might come home from time to time with grandchildren you can rape and feed to goblins, inciting another of those "angry royalty sends army to crush loathsome bastard" scenes. Or you can just train and match-make those, too. I guess.
If you get sufficient numbers of large loyal monsters, maybe you could send them on raids, coming back with animals, women, children and angry warriors following, forcing you to setup guard walls on either side of the compound. Probably drop your brothel numbers if you have guard walls and war going on. Might want to send in stealthier units or something. But better targets could result in royalty that has some fun purple clothes and little crowns or whatever before you mince them and make some corpse starch or whatever to re-energize the poor girls assigned as horse cocksleeves to keep your prize centaur girl stables filled.
Disloyal monsters released on unsuspecting world just result in angry nobles and pitchfork groups. Gladiators could be kept upstairs till their loyalty is high, letting them escape to cause problems ( sometimes returning themselves as raiders/liberators if you were a real dick to them, torturing them to get their fear without sufficient reward to make it loyalty ).
To actually answer, some lamias/nagas ( need to eat, don't forget while they're in the slave pen with normals ), harpies, and centaurs would be good. Maybe some huge amazon chicks that need a large slave pen and large mating room, and who if disloyal will just crush smaller mates.
Not to mention goblin girls and other female variants of the monsters. Raiding parties could come back with all manner of themed slaves to break, perhaps even male slaves if you want to avoid incest-fest or putting your favored girls to public use while keeping the genepool fresh.
Probably result in some gay content in game, but I can always just not do that or deactivate it, no worries. If anyone does the rest of this and a little wiener on wiener makes them lose their minds, they've got some real problems.
It would be pretty funny if leaving smaller races in with the orcs and minotaurs would result in them getting cornholed to death. Those orcs don't look very discerning to me.
Rooms people pay to watch monsters rape folks in would also be super taboo, inspection-wise, but should be high paying as well.