They already made a Final Fantasy movie,
They did neither, and gave it the title of Spirits Within,
It seems more like a nod to how the series is unique in each entry. It doesn't feel entirely off from the newer games since VII either. Which is way more high-tech than previous games. The way the whole spirits thing comes up is similar to the Livestream, and likely influenced by it(unless there's more data on the movie's creation).
My original opinion of the movie is; it's a great movie, it's not just a Final Fantasy movie. But I'd say it still fits fine within the series overall, it's just more its own thing than you'd expect. For instance, with FF being notably turn-based from the start, even with battle gauge, having a completely different action rpg style is a huge change. Being an MMORPG itself is a huge change, which are mainline titles too. To me, it feels like it jumped the shark the second it went too far with the gameplay and changed it up too much. Those are cool for spin-offs, and updating the turn-based to be more intuitive is fine(which is what battle gauge was). It was never a straight turn-based series alone either, as you didn't have a character go, then another character, and so on. You chose people, and agility took course in who hit first, all automated.
In addition, Chrom is the mascot of sorts for Fire Emblem: Awakening. He's on the cover of the game, he's there from the beginning, he's the one a lot of people requested, but originally Sakurai deemed him too derivative, since he'd be just another sword user in the same (literal) vein of Marth and Ike.
Then, someone presented the idea of using Robin instead, who, despite being less popular and having less of a personality/character (as the player character), had way more unique moveset potential, hence why Chrom wasn't added until Echo Fighters were given a distinction.
So it really is a case by case basis. Sometimes the mascot of a game doesn't make as much sense as other options.
True. Though, from what I've observed, not that it's a hard rule, it seems for the first entrant in a franchise, they will use the series' mascot or a mascot from a specific game in the series. It's why I believe both Erdrick and Slime are extremely good choices respectively. And on equal levels of importance. They both are severely important to the series and all.
Keep in mind I never thought mascots inherently could get in from another game over a less important character by default. It only mattered for the first entrant in the franchise itself. I probably didn't clear it up what my opinion of how mascots mattered was. Villager is arguably the only character who wasn't a proper mascot at this point, but still the first entrant. But that might also make him the exception to this style of choice. I forgot about him when saying my belief on how mascots work, to be fair.