Cuckold with my girlfriend's slut

Hey, we're a couple of 25 and 28 years old, she's 18 when we started. We've always been a very passionate couple and never had sexual problems, but since a while ago I got into the world of cuckoldry and it really turned me on from the start. One day while we were having sex, there was a delivery about to arrive and I suggested that it would be cool if she received it in a thong. To my surprise, instead of getting angry, she was surprised and turned on, although she didn't say anything, she just stayed there. From that moment on, our conversations during sex were increasing the morbidness until she alone confessed to me that she'd love to get herself with another man while I watch. This was the starting point for fantasizing about all kinds of men, from strangers, professors and even family members. Our sex thanks to this is incredible, but I'm willing to take the next step. I'm crazy to see a guy take over my girlfriend and see her face in complete pleasure. I'd like recommendations on how to keep going and if you'd like to be part of this fantasy.