I honestly think that if they wanted they could do it all in three collections.
First collection (1.5): KH1FM, Re:CoM and Days cutscenes.
Second Collection: BbSFM, BbS Volume 2 HD Cutscenes, KH2FM.
Third collection (This one could really be 2.5): Re:Coded and Dream Drop Distance.
My reason to include DDD is because normally companies have a certain amount of time of exclusivity. Once that time is over you can do whatever you want with the game. By the time of a second or even third collection, that time will probably have passed, thus allowing them to include the game in the collection.
Also, doing that would allow them to fulfill this "one game per year" thing until KH3 is done or another game comes out.
I know. A "collection" of two games isn't much of a collection, but 1.5 only has two games too.