Fetishes there aren't enough of here... Hrm. Here's a few I can think of.
- Multiple-generation inbreeding. Siblings have sex, make babies, babies grow up, they have sex, make more babies, repeat ad infinitum. The longer-term form of this kink is what is limited. There's only a handful that really qualify here (LT, BotN, MS, etc), and most of them are stuck in development hell. Granted, most of the challenge of this idea is in the coding, as new characters keep getting made. Although, if one wants to write a Visual Novel that goes through four or five generations (or more), I don't think people would mind the lack of flexibility.
- Free Use. There are two forms of this. One is basically just sex slavery, where one/few are forced to service any comers, whether they want to or not (probably not). But the other form is the one that isn't used much. Where people are DTF on the spot, and quite eager and enthusiastic to do so, out in the open where people can see and comprehend it's happening, but don't think it's unusual or care to interfere. It's basically the sex-form of the 'Unusually Uninteresting Sight' trope, if you feel like looking it up on TVTropes.com
- Massive pregnancies, where octuplets is the lower end of the number of incoming babies.
- Multiple-generation inbreeding. Siblings have sex, make babies, babies grow up, they have sex, make more babies, repeat ad infinitum. The longer-term form of this kink is what is limited. There's only a handful that really qualify here (LT, BotN, MS, etc), and most of them are stuck in development hell. Granted, most of the challenge of this idea is in the coding, as new characters keep getting made. Although, if one wants to write a Visual Novel that goes through four or five generations (or more), I don't think people would mind the lack of flexibility.
- Free Use. There are two forms of this. One is basically just sex slavery, where one/few are forced to service any comers, whether they want to or not (probably not). But the other form is the one that isn't used much. Where people are DTF on the spot, and quite eager and enthusiastic to do so, out in the open where people can see and comprehend it's happening, but don't think it's unusual or care to interfere. It's basically the sex-form of the 'Unusually Uninteresting Sight' trope, if you feel like looking it up on TVTropes.com
- Massive pregnancies, where octuplets is the lower end of the number of incoming babies.