Hum... I could see anything honestly. It following a game wouldn't be bad, only hard thing would be to get used to the look of the characters (depending on how they make the movie) Maybe possible voice actor changes, ugh.
Filling some of the gaps of the series would be awesome as well, showing what exactly Riku did between CoM and KH 2 as an example.
An origninal story would be my favourite though I guess. New characters to meet and in a different universe which has nothing to do with the game series. They can't do that much wrong then I guess x3