I think humor is the key. Don't come out and say what you want, but paint a picture of it in her mind. Find some porn videos of what you want. Make sure that the woman in the video is lesser than your wife in looks and body. When the gal is getting pounded by that black cock, tell your wife how sexy the woman is in the video. Then just let that sink in. One day she will ask you if you think she's sexier than the woman in the video. Then you say: "Hell yes! I should have gotten you in porn years ago. We'd be filthy rich by now." Then drop it.