I have the same fantasy, to get my wife thinking about another cock I asked her if I could turn up the heat in our sex play by using a big black dildo on her and she thought about it a few days then said sure it might be fun. I immediately purchased a big black Cyber skin dildo. picked my time to use it and sent her over the moon. She absolutely loved it, she was not expecting the stimulation and pleasure she received from having her pussy filled. Multiple climaxes! She said she never dreamed how good a big cock would feel. So I brought up the idea she should try a real big black cock, she didn't refuse or dismiss the idea, so now I gotta keep her thinking about trying the real thing. Let her fantasize about it and encourage her to follow her need to try a BBC. Keep telling her how exciting and exhilarating the act of fucking a BBC would be for her. I told her it would so hot and satisfying, she really needs to do this. She is listening and considering how bad she wants it. Where the mind goes, the body follows.