Besides (and including) things like the basic faults of the score and design and what's been mentioned:
1) Explore how Miranda is outdated, and why her cruelty is toxic beyond the fact that being mean to people is bad. At the end of the show, she saves her job by screwing over Nigel and sings about how you have to be ruthless to stay on top and not be just a nobody, etc. I would love for there to be either more of a sense that this a Pyhrric victory, or that she's right, in the sense that she's won and will continue winning to the detriment of the rest of us. One of the two would be a proper conclusion and provide some kind of message.
2) Identify and detail what makes Andy tick. She's always been a vague character, and she's vaguer than ever in the musical. Part of this is that the roman a clef element somewhat precludes insight, but also that they've simplified her in the first section to a doofus caricature of a liberal college graduate, so basically everything she says is a joke. This means we don't really care about what she actually wants, and it makes her gradual makeover generic, as we have nothing specific to contrast this new behavior with.
She should have a clear purpose for taking the job besides wanting to make rent, such as believing it'll be good experience or that she can make some kind of change, and then we can follow that idealistic purpose being warped. If she doesn't, then work the angle that you can't just be an innocent bystander in an ugly system, and perhaps connect that better to her ideals at the start, so that, in the end, we can understand what she's learned.
3) Narrow the focus to the Runway offices. This not only removes all the filler business with her friends/partner about how she's changing (we know she's changing, we're the audience, we're watching her change) but creates a more high-pressure, locked-in environment and gives us more opportunities to develop the relationships between Andy, Miranda, Nigel, and Emily.
4) Make Emily an actual person. Right now, she's entirely comic relief, so when she literally slapsticks her way out of the Paris gig it doesn't feel like a dramatic turn where Andy takes advantage of a situation in a way that's newly vicious, which would make a perfect act one closer. We should understand that Andy is giving herself over to a new, unsettling habit of cruelty, and we're poised to see how that plays out in the second act. As it is now, she just kind of bumbles into it (well, Emily bumbles into it for her). She says "Emily wants to go to Paris! I can't do this to her!", to which Miranda says "I guess you're not serious about your future at Runway", and Andy replies, "Ok. I'll go", before leaving to tell Emily. All the thrill of canned beets.
5) Give us a bit more to chew on with Nigel. He's an interesting character, and he's given a somewhat promising song in the second act about being closeted in Kalamazoo and then finding himself when he discovers fashion magazines, and that he puts up with the pressures of the fashion world because he wants other little queer kids to see what he saw in them. It's kind of a shallow song, made shallower by the fact that a lot of it feels oddly jokey, but there's a nugget there that could be explored to further highlight the tragedy of Miranda's betrayal and perhaps even point him up as being misguided, blinded, or misled. You could even develop his character to highlight the fact that, as far as we can tell, he doesn't really have an inner life that we can see - a suggestion that there's more to being queer than the cultivated media image, for example, or that Miranda's never given him the space to actually develop as a person. He sings that he's "seen, finally seen", but what happens after you're seen? We've moved past the gay sidekick as representation, so there needs to be more to Nigel.
6) Figure out Miranda's singing voice. As I noted in the other thread, the Cerulean monologue is a good example of the unsatisfying half-measure they've taken with her. She recites the monologue basically word-for-word from the movie, but she's given a chorus backup. In the film, this is an icy, thrilling demonstration of Miranda's steely mastery not only of the industry that she's a leader of, but of her ability to ruthlessly read and strip other people of their pretenses. It's a moment that everyone knows, and will be anticipating, so going halfway by half-burying it under a burbling chorus only takes away from its power because it's only halfway to a new take on the monologue, which is distracting. It never takes off into its own thing, which it could if it were thoroughly reconfigured into an actual song, but it also doesn't maintain the same power as if they'd just had her do the monologue as is, with no accompaniment. Some might consider the latter disappointing, but it could also be seen as a challenge for the actress.
Excepting her sudden (and somewhat muddled) burst into belting in the last act, the rest of her singing is in a kind of typewriter-lesson staccato that doesn't quite qualify as patter and could use tightening (there's a lot of weird scansion in this show's lyrics). We could either use more development of her character as a singer before she barnstorms, or we could see her entirely as a non-singing character and then her big number at the end is given extra development to transform her before our eyes into a character that sings, in a true grand finale blowout.
7) Do something about Paris. It's not 1955, we don't need goofy accordion music. This could be a moment where Andy realizes that Paris isn't a vacation, it's a even higher-stakes, more suffocating environment. Also, the "in your twenties" song is basically meaningless nonsense. If we're going to have a song where the guy she has a fling with seduces her away from her work, it should have some relevance to the characters. What can he offer her, in this moment, that she doesn't have, and secretly wants? Can he play to her in a way that we know she wants to be played to, and in a way that creates tension by taking her away from something important? Perhaps what she learns from being taken advantage of can influence the speech she writes, making it better, which then ironically assists Miranda in sealing Nigel's doom.
8) We need more from the post-speech confrontation between Miranda and Andy. It doesn't need to be a catfight brawl, just more of a back and forth. You could demonstrate here that Andy has not only become a great, if ruthless, assistant, but essentially a potential equal to Miranda, only that she's seeing a clear choice to use that potential for a better cause. I don't think we got this in either the film or the musical, both of which fall back on letting Andy essentially stumble upwards, rather than actively compromise herself.
We also need more from Andy's post-confrontation climax number. There's no revelation in it for the audience of what she's learned besides that she needs to do things for herself, and not for others, which is the kind of thing you see on inspirational plaques from Bed Bath and Beyond. I don't want to spend two and a half hours in a theatre to learn that I need to "be me for me".
It probably seems like a lot of these notes are aiming for a show that's tenser and more dramatic than might seem necessary, but I feel like it's a better option than going in the opposite direction and making it fluffier. Great musicals are all about tension (and release of tension), and there's so much potential for it in this work that goes unexplored. We need those moments where the entire audience stops breathing and leans forward, so that we can get the flood of endorphins when the tension breaks.
That's all (lol).