The Final Fantasy series started its slow death from what made it unique since VI, which I still think is the greatest of the series. The thing that VI started the beginning of the end was the Espers. Up to that point each character was a unique class and only had special abilities. Once everyone could start using Espers Square slowly widdled away the uniqueness of the characters and the other things that made the series fun. For example...
VII- started the single type of weapon system. Character A has a big sword, character B has a gun for a hand, etc... The materia system made characters less unique, everyone could use magic, anyone could become a thief, anyone could summon monsters, etc... The only thing that set characters apart was the weapon that they used.
VIII- Again the only thing that set each character apart was the weapon that they used. Everyone again could use magic, but each character did have a unique attack that was somehow based on the uniqueness of their weapon.
IX- Went a little more traditional with character classes, unique roles and skills, etc... Since I'm an item hoarder when I play the games I even liked how in one area weapons and armors inverted, where if you had a strong weapon and armor the were actively weak, but weak weapons and armors were strong.
X, X-2- I did like the story of X, X2 not so well. Here we took a much larger turn away from what I've liked about the series. The skills system allowed for truly unmemorable characters as far as abilities. Each character was a blank slate aside from their special weapon that each was strong against a certain enemy characteristic. I have a game where my Black and White mage characters hit harder than who is supposed to be my ultimate warrior, and my thief can cast Ultima spells as strong as the black mage. Here we also lost the fun of wold exploration by losing the airship. Sure, there is still an airship or two in the game, but you don't actually fly them, you just point and click to go to your destination. No more leveling or skill building "Islands Closest to Heaven/Hell" like in VIII.
XI- Never played, didn't think the series would carry over into a MMO world
XII- Again, no more character uniqueness, taken even farther in that you can let your spell casters use heavy armor or heavy melee weapons. As gameplay went on I got tired of the live action battles because instead of being able to sit back and enjoy the action I was too busy trying to keep track of my characters health, trying to figure out why the archer wasn't engaging the enemy, or why the healer wasn't curing anyone. Kind of had the same problem with Arkham Asylum, I'm too busy watching the goons trying to attack to pay attention to the moves that Batman was actually doing, I ended up missing more than I got to see.
XIII/XIV- Haven't played either but they never appealed to me. XIII did away with exploring, period. Whatever they did with that linear gameplay had way too many reviewers up in arms that I didn't even want to play it because of the fond memories of the series. They also did something with the shops if I remember correctly that was a break from the past titles too
With today's gaming, I'd like to see a FFVI-X style game come back but also bring back a few features that have gone by the wayside.
Bring back truly unique character classes. Don't let the ninja become a spellcaster in the way that a mage is, but ninjas can have their deceptive type of "magic" if you still want to include it.
Bring back explorable worlds, enough said.
Take the airship to a whole new level. While flying have visible flying enemies or other monsters for "random encounters" or challenges. Remember Doom Gaze? Create a battle platform for while on the airship. How is it that a fleet of airships with canons on them can't turn the canon around and shoot a freaking bird that has landed on the deck? Add a crash system. We can create these great 3D worlds now that would be great to explore or even have planned crashes as a part of the game. Give the airship hit points and after so many bumps into trees, mountains, land, or water make the ship go down. Be stuck in the area for a certain amount of time until either the ship magically gets repaired or you beat a side quest where you find the parts needed to repair the ship.
I think that I've gone on long enough, it really burns me to see what they're doing to this series that I've liked for almost 20 years.