Let's face it, everything relevant in this thread is going to be a spoiler, and if Sephiroth doesn't mind I'd like to just mark the thread title to say spoilers within. Lemme know, dude.
I'm actually not too overly fussed about the Luna stuff I saw (or did not see) to be honest. They perhaps gave her too much importance prior to the game being released. I mean, I get that she is important to some degree, but I don't see the point in elongating her return just so she can join your party. We already have three other people who join your party as it is. We don't have to have Luna join too. I like the idea that the moment we finally get to see the person we've come all this way for is the moment that she gets destroyed. I mean, initially I wanted to see more of Luna but in retrospect I kind of appreciate the feels we got and don't think we need to elongate that too much.

What I WOULD change...

- More/clearer story regarding what happens with...
--- Ravus
--- The Emperor
--- Luna
--- Ignis (blinding part)

Essentialy the above would cover the finer details of what was going on with them all, and answer most (I like having some imagination and theorising left to players) questions that we were left wanting for.

- More time spent in the World of Ruin. Like, a lot more. Not even half the time we spent in Lucis by any stretch, but enough time to visit these other locations and people we used to know and see how they are getting on, how they have changed, etc.

- Postgame should unlock the ability to add previous guest party members back into your party on demand.

- More background regarding Prompto's personal revelation. Or more build-up. I dunno. It felt very rushed, despite the annoying length of the chapter.

Honestly though, I'm happy with how it is. These are just things that would make it notably better, at least for me.