This sounds really fun. I play mainly families with a more modest lifestyle but I would definitely be up for this! I also think it would be cool to add the option for royals to support a cause(s)- I'm from the UK and our royals all support various charities and are patrons for this. Could be nice to have a variety of options for them to support and maybe a semi-active royal 'career' where they can promote their cause at functions, interact with those their chosen charity supports and other similar concepts. Or even if it wasn't semi-active, it would be a nice addition to have the choice for royals to have that 'career' (they don't have to necessarily earn money from it as they could just earn taxes, so it could be optional). It could even change how the other sims view the royals- negatively if they don't do any charity work, positively if they get extra involved; younger royals could choose to support less traditional causes and this could be met with mixed reactions (reflective of how Princess Diana didn't shy away from taboo but important areas, and how the modern royals are big on mental health advocacy etc!).
Also, maybe like a 'rebellious royal' idea- someone choosing not to be the heir, perhaps because of love or maybe they just don't want to be the monarch, or like the younger generation wanting to move royalty to a more modern version!
But anyway, I'm getting super carried away, there's a channel on tv here that keeps showing royal family documentaries on Saturday nights, so these ideas have popped into my head based on some of those a bit haha. Love the detail and thought you put into your ideas for this! Would definitely be cool to see! :)