I honestly don't feel like Luna needed to be fleshed out much more. Especially not since she had a pretty big role in Kingsglaive, which you really should watch before playing the game. The issue was that 1) people assumed she was a re-design of Stella, whom they in turn wrongfully assumed was going to have a major role in Versus and 2) Tabata and the PR team decided to reiterate how "strong" she was when promoting the game.
The game is not really about Luna. We saw enough flashbacks to know how she worked as an Oracle. What they should have done though was to show more scenes fleshing out her relationship with Noctis. They're by far the least believable "ship" in the entire series. The most direct comparison can be made to another arranged marriage, that of Ashe and Rasler, where she gets multiple scenes to reflect on their relationship and it's made clear that there's love involved. Another fairly shallow portrayal of romance would be Cecil and Rosa, who are already in a couple when their adventure starts and thus the player can just fill in the blanks. Heck, even Snow and Serah have more chemistry than Noctis and his bride to be.
In fact, the more we learn about him and Luna the less likely their supposed romantic feelings for each other seem. They literally haven't met for like ten years prior to their reunion in Altissia, and back then they were still kids. You can only do so much with writing messages back and forth in a notebook. It's obvious they care for each other, but actual love? No, it feels incredibly forced. Especially when Luna has better chemistry with Nyx and Noctis has better chemistry with pretty much everyone else, from Prompto to Sarah from Terra Wars.
But their wonky relationship aside, she does what she's supposed to in the game. If anything, I think her brother Ravus and his motivations could have been better explained.
Aranea is the strong female character in the game that people wanted Luna to be. We get to see more of her in Episode Prompto too, and it's a shame we'll never get Episode Aranea as it would have been canon (unlike the other two cancelled episodes) and according to summaries of the novelisation she's amazing in it as expected.
The main focus of the game is the "chocobros", and their bonds are very well written. Especially when you take the DLC epsiodes into account (which at this point you should). I think they come off as a much more believable group of actual friends than any other main cast ever did. And as for the main story, Ardyn is of course the driving force behind everything and he is also very well developed.
The game is not without its flaws of course, but I don't think the main cast is an issue at all. I would have liked to see more of the minor characters, like Loqi (who would have appeared in Episode Aranea) and others from the Empire. The open world could also have used a few more memorable NPC's and it's a real shame we don't get to explore the continent of Tenebrae and Gralea (at least we got a taste of it in Episode Prompto). I really would have wanted to go there before things went south.