You can talk to her. Tell her how you feel. Tell her that idea of watching her cum on a huge cock turns you on. Plan a night out with her where you get a hotle room. Go back to the hotel room and suggest to watch some porn "to get in the mood and spice things up". Then you turn on threeaome porn MFM threesome. Tell her how hot you think it is. Caress her, lick her pussy, tell her how you want her to be happy. Then fuck her and tell her how hot it is thinking about her with another man and how much she might enjoy it.
Then you repeat that night a month or ao later but progress. Maybe bring a big dildo with you and fuck her with that while she aucks your dick. Maybe while you are watching wither threesome or cuckold porn.
Then slowly but surely you progress into acting out your fantacy with the dildo and using the name of a guy that you know she finds attractive.
Then eventually you start talking about actually including that guy in your bedroom play.
Slow and steady wins this race my friend.