Bestiality, specially male (me playing male) human/anthro on female feral. I always get it the other way around and it can be frustrating. Of course, plotwise is much, MUCH better if the feral is able of some sort of communication, wether it is by talking like a person or a mental link. It is kinda hard to find someone interested in a story with a setting like that.
Living machines seem to be a big no no and it just makes me want it even more! From aeromorphs to dire machines. There is just something about imagining a vehicle you care for spring to life, y'know?
I know it is very niche and kinda weird. But the reason I really like either bestiality or machines in a roleplay is because (at least how I see it) it leaves room for a deeper connection. Something far more intense not only in terms of sex, but in terms of feelings and bonding. Wether it is a deceased beloved one who reincarnated as an animal as a test from the gods, an intelligent and grateful pet, perhaps something more...divine? Who knows! To the bond of human and machine. To me it can lead to an extent of both a lovely bond to very interesting and intense lovemaking!
Something that I also find a bit frustrating is that it is also hard to find size difference, specially larger females. Don't get me wrong, I understand that, comparably, tiny pp won't do much in a much larger female's flower. And that a larger male won't even fit a smaller female without being ripped apart if insertion is even possible. However, when these kind of size differences take place, I notice that people is more keen to be creative when the smaller part is the female, either pps just adjust to the female's size or she just becomes really stretchy. But it is VERY hard to find someone willing to accept or come up with a creative solution for a larger's female pleasure. Something I noticed is that, when a partner WANTS to do something, that writer will go out of the possible way to make things happen relatively organically and with some logic (even if fantasy logic or just winging it). So yeh. I think it also has to do with people actually wanting