I would no start with the concept of cuckolding but rather see what she thinks of being with another man. Maybe bring it up during sex, like...you would love to lick her clit while a big cock is going in and out. See if it gets her hotter. It worked on me way back when we were dating. Keep in mind...videos, fantasies are one thing...seeing your wife in ******* with another man is very different, especially with you in the submissive cuckold role.