First of all, we don't even know if her name really is aqua.
Also I think she may not even be playable a lot of people reckon the other character is Mickey which makes sense perhaps Aqua doesn't have such a big role yet, or, Nomura wants her to be a surprise...
EDIT: Scratch that, I still think Aqua will be the playable character, after watching the FM+ movie it reminded me that she can use Magic... She would have to have a decent role in there somehow I mean she weilds the keyblade too so she can't have an incredibly insignificant role like Kairi, so I do still think she will be the Mystery character, anways I think maybe she could be the most important character that's why Nomura wants her to be secret.