Oh man this is gonna be tough. I've re-ordered them to show you which I chose first and which I chose last. Well, from memory, which I chose first, anyway.

• Would remove the Final Fantasy title (What you think would work best without the FF name attached to it): Final Fantasy XIII. Maybe then people would give it a bit more credit!
• Change a plot-twist: Final Fantasy I, I'd just remove the time travel thing.
• Change a battle system/LVL up system: Final Fantasy II, so I might like it.
• Dump a Final Fantasy: Final Fantasy III, because I've yet to play it for very long and nothing about it grabs me.
• Would remove your memories around it so you can play it again with a fresh mind: Final Fantasy X, maybe I'd enjoy it more with the Japanese voice acting instead of the horrid voices of Yuna, Seymour and Tidus in the English version. I'd have to have a fresh mind so it wasn't always in the back of my head what they sound like in the English version.
• Improve a Final Fantasy: Final Fantasy XV, there are loads of things I'd like to add to this story.
• Make a sequel/spinoff/another spinoff: Final Fantasy VIII, I'd love a spinoff focused on another time with the sorceress being different, and with more information and note given to the Shumi Tribe who are excellent lore-wise with lots of potential.
• Give a Final Fantasy a new soundtrack: Final Fantasy XI, not because the soundtrack is bad, but because I don't really mind if a MMO OST is changed compared to a single player OST. And I didn't play FFXI much anyway.
• Would change the MC into a fluffy lazy cat: Final Fantasy VI. An ensemble cast made entirely of fluffy lazy cats!
• Give a new ending or opening: Final Fantasy XII. I always felt the ending of this game was rather forgettable, even though I've seen it twice I still can't remember much other than Penelo reading a letter.
• Give a Final Fantasy an animated show (like the Persona games): Final Fantasy XIV, because I think an anime following the scions would be pretty epic.
• Make a Final Fantasy easier or harder: Final Fantasy IX - By "easier" I mean "easier to tolerate the incredibly slow battle system at the early stages of the game".
• Would change/improve a dungeon or final dungeon: Final Fantasy V. I remember some of them were kind of boring. *throwaway option*
• Give a Final Fantasy a new protag or villain: Final Fantasy IV, because I've yet to finish it and can't remember a huge deal, so I wouldn't be upset if Golbez was replaced with another villain.
• Keep a Final Fantasy the same (no change): Final Fantasy VII, because it's the only one I haven't chosen yet and I love it just the way it is.