well the differencve between the final fantasy and kingdom hearts series is each game(excluding expansion packs and sequals, i.e Final Fantasy X-2)
the FF series in each game has different characters and plot and magic, different world, system, weapons and that, while KH has the same characters, same worlds(with new ones), same weapons(some new).
KH WILL never be the same a FF, KH is about the keyblade master and FF is about each main character in the game. the only way KH is gonna come close to FF is if Sora dies off and brings in a new keyblade master and a different group, different weapons, new magic like Scan, Quake, Aura. Introduce Blue Magic instead of White and Black Magic.
if you get my drift, but for the series in the long run, i think KH should stop after KH3, if there is gonna be one of course. and make a game to introduce new characters to get you into the plot, like CoM did for KH2, a bridge bewteen KH2 and KH3.