This may have been done, but it popped into my head, and I can't recall seeing it. What if there were a consensual NTR story (netorase, I think it's called, but I always get them mixed up) where the weak guy explicitly has a cuckold fetish, so the couple brings a guy in. It plays from the bull's perspective, but as a twist on the usual dynamic, the cuck and the gf/wife are still in a very happy, loving relationship. The bull starts actually falling for the woman, and has to deal with the notion that ultimately, he's just the meat. In a sense, he's getting emotionally cuckolded despite being the one doing the fucking.
The problem is, I'm not sure who this would necessarily appeal to. It's from the bull's perspective, which wouldn't necessarily appeal to normal NTR fans, but it also features the MC being the one feeling loss and sadness about the relationship, which would turn off people who prefer the dominant perspective. Maybe it wouldn't work at all, or maybe it's been done more than I realize, I just thought it was a mildly intriguing concept.
The problem is, I'm not sure who this would necessarily appeal to. It's from the bull's perspective, which wouldn't necessarily appeal to normal NTR fans, but it also features the MC being the one feeling loss and sadness about the relationship, which would turn off people who prefer the dominant perspective. Maybe it wouldn't work at all, or maybe it's been done more than I realize, I just thought it was a mildly intriguing concept.