Well, I know my charries already have a background story. Perhaps, the Queen calls for assistance from her allies in other areas to come aid her in seeking that damned blade. That can lead into something, which leads them into something else or another person's subplot.
Celestia will possible be traveling to seek aid from other kingdoms already, as well as looking for scholars and priest who might have knowledge of the Diadem of Dragons. She needs answers to several questions on her mind about how to stop the prophecy from happening, and will go through great lengths, even without her mother's knowledge, to find the answers to them.
As for Nahra, she'll do something off the wall, ignorant and flat out childish in order to get out of the house, unaware why her mother and sister are psycho about her staying inside. I'll possibly have her run into some caravan or a group leaving the kingdom to seek adventure or something, and decide to jump in and travel with them, thinking she'll be back before dinner that night. She won't, of course, so that'll lead to another huge problem that must stay under wraps. There's not a lot of people who know who she is, but the few who do are hunting her. Therefore, the Queen has to reveal to a selective few that her daughter is missing, without alerting the entire countryside.
Figured there will be several ways to connect with other charries and their subplots, and possibly glue all of them together to connect to whatever huge, grand plot for the entire story later on. That's one of the coolest things to do - take random subplots and suddenly come up with ways to combine them to the background plot of the entire story; they become situations that either help or hurt the main plot, but in the end bring about success to the protagonist group and a horrible end to the antagonist(S)... which, I figured by the time we're done with this entire thing, they'll be several mini baddies, and one huge one for the entire story.
Um...I really need to go to bed, but I've got to stay up for my friends who are traveling from out of town to stay with my and my fam for the week. So, if there's something in all of this that just doesn't make since, I have a good excuse.