If you are really planning to be castrated or castrate someone else I'd recommend getting some mental health assistance immediately. This extreme type of self harm would be life threatening and likely criminal for anyone you'd care to involve. Sorry to be a killjoy but when reading this sort of thing I feel compelled to add some sort of caveat before commenting further, even when it's posted in the most playful, fantastical sense,
Okay now that that's done..here's what I think about the FANTASY of being castrated. Bear with me....
I once saw a documentary about a prodomme who spoke about clients who enjoyed scat play. Scat is one of those things I've never been able to understand. Why would anyone want to play with shit? I wish I could find the link to this film (any help here?) but the prodomme did a great job of explaining the attraction. Of all the taboo things you can do, scat might be among the most extreme. We are deeply wired to have an aversion to our own body waste and that of others. To cross that boundary WITH someone else feels (as the prodomme described) intimate, exciting, and dangerous. It was the breaking of the taboo that was exciting to her client, not the scat in particular. I finally felt like I had a bit of insight in to the fetish, although I still have zero inclination to explore it myself.
Castration fantasies are also extreme. Dangerous. Intimate. Forbidden. They arouse all sorts of emotions. What would it be like to be so utterly and irreversibly emasculated? I get why it's an interesting place to go in your head.
As far as exploring it in roleplay, I once saw photos of a guy who was quite heavy tape up his cock and balls in a...sorry no nice way to say this...roll of fat. And then his domme took pictures for their entertainment. Maybe something like that might be fun. But don't forget, sooner or later you might find you need your balls for something or other. Best to hang on to them