You guys already have a scientist using the Zil make pills for honey flavored bodily fluids. but there are so many races with interesting characteristics that can be added to the player character locked behind random chance from their items. What if you added extremely rare versions of each of the other races, sort of like Alpha's of each race that only appear very rarely like 1 for every 10 or 20 of the others you encounter who can be captured for pills to give you their various racial aspects. But so you can't get all of them at once, because of them being exceptional specimens of their races, not like the common variety Zil you brought Doctor Haswell before, they are able to escape before he can make more than one, facilitating the need to capture them multiple times, thus making it not so easy to get all that you want.
Or if that doesn't work, maybe have them drop an extremely potent but limited version of the transformation item that the doctor can use to make medicine of one of it's effects but because of how small of an amount it was he could only make one type of medicine thus needing to find them again to get more of the item.
Or if that doesn't work, maybe have them drop an extremely potent but limited version of the transformation item that the doctor can use to make medicine of one of it's effects but because of how small of an amount it was he could only make one type of medicine thus needing to find them again to get more of the item.
