Two things would be a great help in my opinion.
1) Go back to open world, or at least the illusion of open world, in the case of Final Fantasy X. The reason I say this is because something that I utterly loved in the PlayStation era of Final Fantasy was exploring. Finding areas on the map that you could completely miss if you chose to simply follow the main story. Who remembers hoping into Ragnarok in FFVIII, flying around in the middle of the ocean, and suddenly finding the Research Island in the corner of the map, even though you map said nothing was suppose to be there? The games need to bring back that feeling of exploration. Yuffie and Vincent were completely optional in VII, yet if you found them, they weren't just tacked on characters. They had their own full stories. Half the fun of that game was wandering around, finding a place that made no sense because you didn't have the right idea/person/whatever, and then coming back later and going, "Oh, I get it now!" Which ties into my second thing...
2) Give us back our side quests. This started all the way back in Final Fantasy XII. Side quest were completely scrapped. XII only had the hunts, and that was it. XIII had hunts, with the illusion of fighting a truly epic boss at the end of them (That giant rock monster), but then totally bailed on that reward. XIII-2 at least gave us some side quests, but none of them ever felt epic. Remember going to Yuffie's village the first time? Remember how that little side trip turned into a huge epic quest? Remember in VIII when you tracked down Ultimate Weapon, because suddenly the original side quest you were on branched into another one? Or just doing all those silly things in the library just so Zell could get a girlfriend? Remember in X when you walked into the cave where, again, Ultimate Weapon (or whatever he was called in that game) was hiding and you were treated to his story? Remember in X-2 when you went on a sidequest to figure out Pain's story? Remember doing all those choco games in XIII just so you could get to Ozma? Remember when side quests helped to expand the world and offer stories beyond the main one? How they helped flesh out the characters and revealed bits of history from the world the game was set in? What did XII give us? Oh, hunt down monsters so you can be the best hunter. Oh, and a little bonus clip about how the hunts were apparently set up by moogles for revenge, something that is never even hinted at until then, and just dropped afterward. XIII gave us the realization that nature needs to maintain balance. XIII-2 gave us multiple side quests, so it took a step in the right direction, but none of them ever ended up meaning anything. In fact, a lot of them ended up being rather depressing in the end, or just raised more questions.
So yeah, those are my two suggestions on how Final Fantasy can pull itself out of the rut it seems to be in. I think XIII and XIII-2 took a small step in the right direction (I really don't like XII), but it still has a long way to go before it's back.