The more I think about it the more I kinda wish that book 2's slave route placed you in the body of Azula's father instead of some rando. There's a HUGE pile of daddy issues to dig up and play around with, there would be many potential ways of disciplining her if you know what I mean, you could still keep the comments about royalty and incest in book 4 should it include knocking Azula up(with a slight name change on who is the father of course), and there might actually be a use(and the resulting consequences to play around with) for the morality system when you have the influence of someone as far-reaching as the firelord. Honestly it's a goldmine.
The only issue would've been how you would go about learning firebending as someone who is supposed to already basically be the best at it but book 4 managed that with its slave route so it's not like it's completely out of the realm of possibility.
Of course, even with a mod it's unlikely to ever happen at this stage as it would be quite the ambitious project to say the least, but, well, the idea is there.