Hi. I don‘t know if I can help you, since to give advice on how to best approach, I would need to know your wife, but I‘ll try. Do you two watch porn together? If so, maybe an „accidentally“ light kinky porn could tell you if she‘s interested. Or you could watch a movie together and then talk about the movie. There are some with BDSM context. Or you could give her this book https://www.amazon.de/dp/B005HZ6FH0/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1 and see how she reacts.
But actually, you know your lady best. In a relationship people should at least be open to their partner‘s interest and non judging. Maybe writing the stuff down and giving her the letter could help you to get a conversation going. And if she starts kink shaming you, would you then really want to still be in a relationship with her? 🤷 I mean, I do get your concerns and worries, you don‘t want to destroy something that you hold dearly in your heart, but do you really want to hide a part of yourself from her? Maybe she‘s secretly kinky as well. Who knows.
https://www.amazon.co.uk/dp/B005HZ6FH0/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1
This book will help you understand more and deals with difficult issues like this .
Bio :: About one out of ten Americans is 'kinky': they may enjoy bondage, dressing in special clothes that turn them on, spanking, erotic role-playing, body modification, or any one of a number of other activities that are outside the sexual mainstream.
And each one of them is surrounded by a constellation of friends, lovers, parents, children and co-workers who may feel frightened, concerned, hurt or bewildered by lifestyles they don't understand.
Now, for anyone who's ever overhead a conversation, glimpsed a toy, or been startled by a tattoo, there's When Someone You Love Is Kinky, a sympathetic and comprehensive handbook for helping you understand the behaviors and lifestyles of the people you care about. Therapist Dossie Easton and writer/sex educator Catherine A. Liszt team up once again to help allay your fears and uncertainties and build bridges of communication that will last a lifetime.
I don't see this mentioned in other comments -- you might want to try a "Yes, No, Maybe" list. It might seem awkward or corny, but if you are comfortable with these kinds of conversations, the list itself can be a good time.
Googling will get you everywhere for pre-made lists. You can also make your own. Here's a random collection of lists I found but haven't read.
Hope that's helpful!
EDIT: It's been pointed out to me that you are specifically talking about fantasy. If you don't actually want this stuff in real life, I think it's completely up to you whether or not you disclose your private fantasies.
If you do disclose them, you have the option of making yes/no/maybe lists for things like watching porn together, and what kinds, if you're into that. But that is still different from acting things out in real life.
ALTERNATIVE EDIT: If you actually do want to move towards acting these things out, it's also been pointed out to me that there's a book about this. Good stuff in Chapter 6 about negotiating kinks, including how to do self-made Yes/No/Maybe lists. The book is called When Someone You Love Is Kinky.