People just want another Lightning.
No, they just want Stella.
I've watched the trailer and I've been keeping my Luna opinion to myself up until this moment. From a woman's point of view, here are my two cents about her.
Do I think Luna is weak? No, but I don't think they're properly highlighting the traits that make her an independent or a strong character, especially in Kingsglaive, but even then you can see signs of her not being your average spoiled princess. She has a role, a duty, and a purpose to exist for and she knows that very well.
She might not have the fighting ability that Noctis has but so far I've seen her show more emotional strength and determination than Noctis ever did. She surpasses Noctis in being emotionally independent and aware of what she must do in order to save the world and prevent it from falling in an everlasting darkness, while Noctis is literally just moping around having no idea what to do to the point where even Gladio calls him out on it and actually calling him "useless" despite his powers and abilities. As opposed to Noctis, Luna is actually loved and respected by the public as an Oracle. If anything, the footage I've seen so far paints Noctis as the spoiled prince, not Luna who has seen far more shit than Noctis ever did.
Her emotions and feelings towards Noctis doesn't make her weak and I find the notion of that is ridiculous, as being strong doesn't mean being immune to love and care for someone for a long time (especially someone who she kept contact with during her captivity, its only natural she'd develop such feelings for him). Also, we haven't really seen her interact with Noctis, so we don't know if she acts like a doormat around him.
As for Luna being slapped or being abused since childhood, the very fact that she still held her ground despite all of that shows a great deal of strength to me. The thing is, there are a lot of ways a character can be strong and Luna displays more of an emotional strength and independent mindset than your typical kind of strength that a character like Lightening shows for instance. Even Aqua, the character people praise for being strong, had her fair share of weakness and was almost impaled by Ventis but that didn't make her a weak character because that's not what classifies a character as weak.
What I am trying to say, is that she isn't another "Princess Peach", she isn't there to be a trophy to Noctis. She is a respected Oracle with a very important task of holding back the Plague of the Stars, who bravely stood her ground against Leviathan and the Niflheim.
The game isn't out yet, she might end up a very underwhelming character or she might actually surprise everyone, but saying she is weak based on few incidents while ignoring other things is not fair.