Interested in hearing others' kinkiest relationship fantasies. I'll go first...
My most taboo fantasy (and it's a long running one) has been having a long-term relationship with a cuckold couple in their 40s or 50s where the husband is allowed some helpful service roles like providing oral pleasure and massages between fuck sessions, being invited to rub and squeeze balls/nipples at key moments to help facilitate orgasms, and, when asked, helping to physically dominate the wife (holding her legs back or holding her down during penetration, guiding my cock into her, etc.).
Apart from just liking the idea of playing with a wildly kinky couple, I like the quasi-familial taboo dynamic of an older couple taking a younger man into their marital bed. In the context of such a relationship, I get wildly turned on by the idea of regularly visiting to pound the hell out of the wife while the husband takes pictures or otherwise helps out with kink play.