I have a play-within-a-play in my novel (or, more properly, the characters are putting on a play). The play isn’t central to the action (I originally inserted it in order to give the characters an excuse to gather regularly), so I hadn’t thought much about its content until now.
Inevitably, whether I want it to or not (I want it to), the play will comment on the novel, but I don’t know at this moment what those comments might be, nor how covert, nor which devices might convey them. And I’m not going to try to figure that out (at least, not right now). Instead, I’ll flesh out the play to serve my characters’ needs (because, after all, their needs come first), and in the process I expect to discover connections that they care about.
Mental parentheses on my part.
That’s a good method; focusing on the characters rather than the plot. I find that those sorts of things tend to make themselves apparent as the novel grows. There are always those loose ends which weren’t in your original plan and now need some kind of emotional anchor in the story. Good luck with this!