The little ones
What if instead of Miranda already having Caroline and Cassidy during the Devil Wears Prada, she becomes pregnant not too long before Paris Fashion Week?
When Stephen finds out, he is furious, never having wanted kids, so he files for divorce as he does in the movie. Miranda is heart-broken, a bit younger than canon (having recently turned 39), and feels as if she has set her unborn children up for failure (according to research some twin births can be detected as early as six to seven weeks, so it’s up to you how far along Miranda is).
Enter one Andrea Sachs, Miranda’s 27-year-old assistant who cares deeply for Miranda and walks into her suite to find her distraught (like in the movie). The catch her is that Miranda is more emotional (she’ll tell you due to hormones but honestly it’s because she’s a real softy and has grown to care for Andrea, considering her a friend though she’s yet to admit that verbally) and with a little prompting, she breaks down and unleashes her worries onto Andrea.
Andrea is furious with Stephen, but assures Miranda that the twins will be find and that they (meaning her and Miranda) can handle. Miranda needs this reassurance and to know that someone else cares more than she’d like to admit. The two form a bond over this, but Miranda turns waspish for a moment, telling Andrea that she cannot promise to be there going forward until after tomorrow (this is because Miranda feels that what she is about to do to Nigel will cause Andrea to turn against her). Andrea tries to reassure her that whatever is going to happen is not going to change anything, but Miranda feels differently. Still, the two move forward with plans, even though Miranda is very nervous that after the next day she will be navigating motherhood without anyone by her side.
Nigel does not react as well as in canon at first though he eventually does, but Andrea is still by her side from the onset. What happens after the announcement of Nigel being passed over is up to you.