Try mentioning it whilst on a night out (as a joke) and see what the reaction is, if it's positive then you have the chance to make it a playful situation, "but don't push it to far, don't expect her to cuck you that night", play with the idea whilst you are out, then take that idea to bed with you and talk about it whilst having sex. if the response is is not good then don't push it. if the response is good, then talk to her afterwards about it ask her if she has fantasy's that she wants to fulfil and let her know that you are open to this.