Dataminers found Luna as an unfinished playable character in the game, suggesting she was intended to be a party guest like Cor, Aranea and Iris at some point. Haven't read any details about it, but I'm guessing she'd play more like Noctis during chapter 13 with the ring.
I'm over her lack of strength at this point. She's a plot device and I've made peace with that. I think my biggest issue is her relationship with Noctis. Their marriage is an arranged one, and they've not met each other for ages so how could there be true love involved? Yet, the game wants to push that idea on us. Their relationship feels incredibly platonic. I don't see how you could interpret it otherwise. Ashe's marriage was also arranged and given just as little attention in Final Fantasy XII, yet they managed to actually convey that she and Rasler were actually in love. Taking the game and it surrounding media at face value, Noctis has a stronger bond with Prompto.
I would rather have had Episode Aranea, fleshing out a character that's already got a solid foundation and while she got some more development in Episode Prompto it'd be nice to have some more. Her episode would also have served as a good excuse to get a closer look at the Niflheim Empire and its leaders. Hopefully that's part of Episode Ardyn, which is set a few decades before the game starts. Would love to actually see how he played the lesser villains like a fiddle.
Episode Cor would also have been nice, but I guess we learned most of his relevant back story in Episode Gladiolus. This game has so many interesting characters that needed more screen time. That and the pacing of the plot are its two biggest issues. I'll never understand people who say there was no plot, or that it was hard to follow. It's actually very simple and straight forward, and I dare say classic Final Fantasy. It's basically a combination of VI and X.