I want my gf to worship my ex's feet

I absolutely feel this, when I told my girlfriend about my ** she was intrigued as she'd never done anything like that before. Eventually, instead of me licking her feet all the time, she started experimenting with mine, and it was the best feeling in the world, having a woman under your soles. She loves it. She even lets me leash her up, tie her up, act as her master whilst she worships them.
However I can't help but fantasize about her worshipping another man's feet in front of me, perhaps a bigger man, more muscular, older but not too old. I don't want to do cuckold play, there's a certain weakness I feel in letting your relationship get walked all over by a complete stranger, and I'd feel ashamed if I let it become reality, but I can't help getting off on the idea. If it did happen, it'd have to be on our terms, and with someone we can trust. The way I envision it, she's getting dominated the same way I dominate her, making her a foot slave pet, and also having ** ** with her and making me watch the entire thing. I don't know, seems kind of wild, I've never told her I feel this way, I don't think I plan to either. Although we have discussed **, and she gets super excited over the idea, as do I.