I'm not trying to yuck anyone's yum, but I feel like some of these suggestions aren't very fitting for a story like this. Some thoughts:
In regards to femboys, twinks and other young bulls, they're relatively common, but I never find that they work for me. Their dynamic tends to be a milf x shota one, where the girl ends up doting on the inexperienced male, and that does not feel arousing to me in an NTS dynamic. I don't get what the husband gets out of seeing that. The wife never feels insecure. There's no inherent tension, outside of just the basic jealousy that the kid is getting some while the husband isn't. They also tend to devolve into the boys being immature and pushing their luck too far and doing things that break the agreements. Overall I just don't tend to like these. It's because of this I think Marina 2 is better than 1, and the Dojima route of 僕に抱かれ喘ぐ妻は is better than the Naoya route.
Piercings, tattoos, and the like, fall heavily in the humiliation camp for me. There is an aspect of corruption to them that veers uncomfortably close to NTR. I feel like NTS is fundamentally a vanilla fetish. It safely explores the fetish in a way where they can always back out of it if it gets too uncomfortable, and have their happy ending. But a tattoo or a piercing are permanent marks on her body that can't be undone. It's no longer just the titillation of the husband seeing his wife have sex with another, but the feeling that she'll never be fully his anymore. Also like someone said above, I just find these elements look visually very corny and ugly.
And interracial I feel veers too close to humiliation too. 99% of the time it's used expressly for that purpose. It's a giant dude that brutalizes the girl and makes it so she can't enjoy having sex with her husband again. It's often paired with dick size humiliation and the bull doing all these underhanded things to steal the girl. Personally, that ruins the NTS for me.