I don't know if you are aware of it or not, but now a days it's a very common fantasy. Even people go much more beyond this. Couples are happily enjoying threesome and orgy. Some cuckold husbands, I'm in contact with are greedy to make their wife agree for such acts.
Even women enjoy this act, some women enjoy in front of their hubby, some at the back. So it's quite normal to have such fantasies. You can slowly convince your wife by changing the way she thinks. Actually most of the ladies are still conservative but women are more flexible in their views they can easily change so keep trying.
There are various ways to do so. There must be some parks in your city where people do kinky activities with dont care attitude, take her to those places, do PDA. Tell her your fantsies fearlessly, encourage her to express her fantasies and make her confident that you would not judge her.
Show her more porn and read erotic stories of your fantasies. Gift her revealing dresses and encourage her to look sexy. How these ideas will work, you can ask more if you want to experiement more.
Hope you will have success soon. All the best.