My biggest fear definitely has to do with the story. I know they'll do a great job, but it has me worried in some aspects. We know Sora, Donald and Goofy are searching for the key that unlocks hearts, while Riku and Mickey are searching for the Guardians of the Light.
My main fear is that they'll struggle with developing the characters. With all the rumors and news we've heard, tons of characters will be appearing. I mean we have Sora's group, Riku and Mickey, TVA, the 13 seekers of darkness, the guardians of light, the "lost" (Roxas, Namine, Xion, Ansem the Wise, ect), plus the likely inclusion of Final Fantasy characters. That right there is almost 30 characters. That's not even counting the Somebodies (Ienzo, Even, Dilan, ect) and possible returns of Marluxia, Larxene, Luxord and Demyx (and their respective Somebodies.)
That's my current fear.