Oh boy, this may be long...
Battle System:
Bring back ATB, it's not dead and it offers more in terms of strategy than what you've been coming up with for the last couple of generations, the problem has been the fact Square has been making the games too easy so they stay accessible for newbies. Yet series like Demon's Souls/Dark Souls, and MegaTen have shown there is a market for challenging games. You don't have to make it as hard as those games but it doesn't mean I should be using basic attacks to win every battle. So stop with the health restoring save points and auto heal after battle, make items harder to acquire and more useful, bring back enemy gameplay variety with enemies that are immune to physical, ones that have brutal party killing counter attacks for abusive skills, boss fights that require more strategy than hit it with my limit breaks/summons. Also ditch the A.I. or at least make them optional again like XII (just give us a better interface if we choose to ignore it) cause if we slow the games down a bit we the players won't need any of that nonsense.
The other strength of using ATB is that you can make the cinematic in battle more focused. The issue with XII and XIII is that there is so much action going on the screen you can't really pay attention to the finer details. ATB would allow for the artist to go wild with cool animations and special spell effects, just make sure we have a skip function for them if we don't want to bother. Final Fantasy used to be a series known for being cinematic and I feel the most recent entries have made us feel more like an MMO or action game.
Use X's version of Summons, it was a good idea that was marred with shoddy balancing, but I feel the idea could create so much more potential for strategy if you just played with it more. I also like the Summons being something more than just a ten minute Black magic spell so let them be playable and customizable.
In terms of customization systems... Part of me wants to say FFTactics style job class system but another part of me would like multiple elements like VI, have the characters have unique classes and stats but then also give me the ability to mess around with them. I'm thinking of something in line with what you could do in Breath of Fire 3 and 4. This way, my characters can be unique outside of personality and limit breaks but also give me the ability to really shake things up if I want.
World Design:
Dear Squenix, I need your employees to sit down with a Wii and play Xenoblade (you know, that game made by former employees for your company) and then I want you to play Dragon Quest VIII (you know that game series you own but you hired someone decent to make it for you) and now I want you to merge those ideas together.
From Xenoblade, I want the massive world I can explore and by explore I mean go almost anywhere. I want to jump, and I want to swim and I want to fall to my death sometimes but all of this makes the world feel so much more alive. You need to take this games scale and up the ante. From DQVIII I want you to see how it's possible to still do world maps, give me my expansive world I can walk around and explore, but when I hit ocean, I want a boat or an airship and I want their to be an old school style map.![]()
Love story:
Make it a side story and don't let it overshadow the main plot. I was fine with VIII doing it cause it's theme was love but then you did it in FFIX and X as well so no, bad Squenix, this isn't Hollywood where you have to have some kind of boy meets girl love story crammed in whether it makes sense or not. Speaking of which, can we do a game without the boy meets girl love story? I honestly feel it would be more appealing and work better for writing to do a more mature story, perhaps about an established couple falling out of love, or perhaps former lovers brought back together due to story circumstances, now trying to deal with each other and their past relationship with each other. You finally gave us a story about the bond of child and parent from the parents perspective in FFXIII, can't we finally get a love story from two characters who didn't just finish puberty?
Characters:
Whatever you do, don't do a spunky young girl/boy, don't do a dark brooding loner, don't do a calm, apprehensive but strong willed (while staying meek) female lead, if you're going to do a non-human character, at least let them play more of a role in the story, I often feel like they are only there to be the token fantasy race requirement.
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I'll post more as I think of them...