And people wonder why the word "trilogy" has started making me laugh like a Batman villain who's just escaped from Arkham Asylum.
One of the things that's really fascinating about working at this sort of remove is that I have time to actually test my rules for functionality and long-term stability. To go with an example everyone's likely to be familiar with, look at Quidditch. Anyone who thinks about the rules for too long will realize that they have some pretty serious issues as written, but is that really the fault of J.K. Rowlings? No. She had no way of knowing that her weird little wizarding game would get the sort of scrutiny it did, and it probably seemed like a good idea at the time. (No, I don't expect to get her sort of readership. Not that I'd complain if I did...)
Right now, I'm stress-testing the fae marriage laws. At their most basic, they look a lot like mortal marriage laws: two people decide to get hitched, break out the champagne. And then they start to get complicated. For example, there aren't any social stigmas against group marriage (some fae races practice it as a matter of course, like the Centaurs and the Gremlins) or same-sex couples. Divorce when there are no children is literally a matter of going "I don't want to be married to you anymore" and posting an announcement at the hall of your local liege.
Divorce when there are children requires waiting for the children to reach adulthood, and then asking them to choose which family line they wish to belong to. Children of divorced parents can only inherit from one side of the family, because the other side must remain available to any potential future descendants (ah, immortality). (Kate points out that this probably leads to a lot of people assassinating their parents so as to inherit everything. Kate is very correct in this assertion.) This also means that the parents of a missing, elf-shot, or otherwise unavailable child must remain married until the child is either located or declared dead.
Marriage to a mortal (IE, "playing fairy bride/bridegroom") has no legal standing in Faerie (hence why changelings can't inherit), and thus doesn't interfere in any way with an actual pre-existing marriage, or prevent getting marriage. It's actually not uncommon for fae couples to fight, huff off, marry a mortal, and get back together twenty years later, having never legally been unfaithful.
World-building. It's not just for continental drift and evolutionary pressures anymore.