I would love for the 6-person partying dynamic to once again be the main source of leveling! With Fields of Valor, Grounds of Valor, and Abyssea, the game no longer requires skill. It's too easy. Players are so focused on getting better gear, rather than learning how to play their job correctly. Personally, I think the minimum level to enter abyssea should be increased to 70~75, and Grounds of Valor and Fields of Valor should be heavily nerfed; perhaps back to the one page per/hour Fields of Valor initially had.
Also, many of the newer two-hour abilities descriptions were simply sub-par. Instead of adding new ones, why not add the ability to augment the existing ones upon reaching level 99?
Lastly, as I'm sure all of Square Enix has heard dozens of times, more jobs! I personally think Mime would be splendid, or many of the jobs available in other Final Fantasy games; specifically those from the Tactics series. Just my thoughts.

The best of luck to you!