Apologies, but this will be a long mess but I think this topic is complicated and not well-developed on the site, and I don't consider myself an expert in the terminology but I think there's a lot of specific phenomena and kinks that could be tagged that have not been or have not been tagged consistently.
Firstly, reverse_cuckolding has two images with two variations each, and neither seems to use it to mean the same thing and I'm not even sure "reverse cuckolding" is a term that could very accurately mean something. The word "reverse" has an ambiguous meaning unless it's simply set to mean one specific thing.
The [url=https://e621.net/posts/2985307]first set of images[/url] seems to feature a character having sex with the wrong partner - Not sure if this is it's own specific tag beyond being surprise_sex and mistaken_identity. Perhaps also accidentally_straight, gender_confusion
The [url=https://e621.net/posts/3184375]other set[/url] features a male planning to fuck another's partner, but it's a fake-out, (like a sort of double-cross). For this scenario I might even think a term like "anti-cuckold" might be more immediately understood as a fake-out cuckold scenario, which I'd never thought of before but I could see the interest in it.
As it stands now, cuckold and cuckquean are tags on their own, but there many other terms which have no sort of tag at the moment which may be useful.
Common
cuckoldress / hotwife - The wife of a cuckold
bull - Has sex with a female in a committed relationship
cuckcake - The female equivalent of a bull; has sex with male in a committed relationship
stag + vixen - Similar to cuckold + cuckoldress, but it seems that cuck/cuckoldress is a sub/dom relationship whereas stag/vixen is equal.
troilism - Sexual gratification from observing partner engaged in sexual activities with another person
Less-common / More niche
bull-hopeful - A male attempting to woo a female he knows is in a committed relationship.
(??? - cuckquean-(-or/-er/-??) - The husband of a cuckquean (No term for this in use?)
I don't really think there's inverse equivalents to these dynamics - i.e. I don't think there's a term for say a submissive man who reluctantly engages in sex with a partner of their dominant wife's choosing.
Another scenario I had in mind which I don't believe has its own tag would be one akin to forced_partners where the bull fucks the cuck for his partner who is actually more interested in pairing others with their husband rather than having sex with said others. I don't think stag or vixen relationships would ever get use out of this sort of tagging though since they don't have the same power dynamic in the first place and it wouldn't be noteworthy or unexpected for a stag to join in. Maybe reluctant_partner for this sort of scenario?
Again, I'm not too sure about whether tags for these scenarios would be all that useful to the community as a whole or how to name them, but some ideas I could suggest might be the following:
forced_infidelity
forced_cuckolding for when a male character forces their partner to have sex with another.
forced_cuckqueaning for when a female character forces their partner to have sex with another.
reluctant_infidelity for when a character encourages their reluctant partner to have sex with another
reluctant_cuckolding for when a male character encourages their reluctant partner to have sex with another
reluctant_cuckqueaning for when a female character encourages their reluctant partner to have sex with another
bull_on_(cuckold/cuckquean)
bull_penetrating_(cuckold/cuckquean)
bull_impregnation - A bull impregnating their partner
cuckold_stepfather / cuckquean_stepmother - Stepparent of offspring of partner and another.
(cuckold/cuckquean)_stepson / (cuckold/cuckquean)_stepdaughter
Other terms that are in the same sort of ballpark but are a bit different are the following:
candaulism - exposing a partner to other people for voyeuristic pleasure. (Possibly should imply "forced_exposure") (In use on the site, but only tagged on one image at the moment - Pretty sure I've seen this in several images before though)
Also, I was wondering about the term "child" in a familial sense. Should progeny be used in place since the tag for child is specified to an age here whereas we tend to use it synonymously with offspring or progeny in everyday speech?