I just finished up Final Fantasy XV to the level I wish to complete it (gave up on Costlemark postgame dungeon, not even going to bother with Pitioss). While I enjoyed the game a lot, there was something subtle about the game I disliked.
As many reviews have stated, the story and characters were on the weak side compared to many Final Fantasy games. There were just too many story elements that were either cut, skipped, or not fleshed out. Perhaps some of these will be fleshed out in DLC, some may have been covered in the movie "Kingsglavie", but it just made the game not feel quite that fulfilling. Here's some examples.
1) Who are the main characters?
This is obvious, but the game threw your entire team together at the very beginning of the story without much backstory. This isn't a bad idea by itself, but it felt like the background of the characters was never fleshed out. You get a few conversations in hotels/campsites, but it's not much.
A good counter example is Dragon Quest VIII, one of my favorite RPGs of all time. The characters are relatively nameless at the beginning of the game, but little side adventures give you more information about them as the story advances. I think of the Mass Effect games are another a good example. You get character side missions to help you learn more about the backstories of the characters and why they are there. Final Fantasy XV could have had similar stories, helping your characters deal with "personal issues" in the story. Perhaps these "side stories" will be in DLC later.
2) Aranea and Ravus, no boss battle
In many games, you meet a tough boss early in the game but get out of the fight due to some story element. Perhaps the boss was simply distracting you for some other purpose. You suspect you'll battle the boss later on as the boss was clearly too tough for you earlier in the game.
This is what I suspected when I first fought Aranea and encountered Ravus. Aranea sort of becomes an ally (kinda?). Ravus only appears in a disappointing zombie form later on. It felt unfulfilling.
3) Ravus switches sides
For no reason at all, Ravus appears to switch sides at some point, which seemed out of place, and with little explanation in the story.
4) Prompto being an MT
Late in the game it's revealed that Prompto was a MT. This wasn't really discussed further.
5) What about all the enemy generals?
At several points in the game enemy generals appear. There's Verstael, the main general, which you never really see outside of a cameo. There's the general you capture for a bit in Chapter 6 when you get back the Regalia. He "escapes" at some point. It could have been a good side quest to track him down and get him back and interrogate him. Perhaps leading to another side quest, lets say a secret base you get to take out. Nope, none of that happens. Basically all those enemy generals you see in the beginning disappear and are never seen again.
6) What about the bad emperor?
Similarly, after having a cameo early on, he doesn't appear again. Only in daemon form (which I learned online after defeating the daemon).
7) Universe history
The game could have fleshed out some of the world's history a bit better, for example the history of "the six". The main characters could have had a side quest to some random
memorial/place to learn about history in the universe at any point in
time to flesh out the universe (Ignis could have pushed the matter, as
he's the intellectual one). It's there they learn about some mysterious
history and .... tada Ardyn is the person in the story. If such a quest would have foreshadowed the ending too much, fine. They could have had random quests to just learn general history of the world.