MY THOUGHTS EXACTLY.
Imagine how much better Dream Drop Distance would have been if it was Riku and Kairi going through The Mark of Mastery. Sora is requested to take part, but he declines because he wants to learn how to be a true Keyblade Master through his own adventures. Kairi gets developments by having her conflicts unravelled in the Dream Worlds, and realises that being a Princess of Heart is not a gateway card to get everything handed to her. Riku goes through his arc of overcoming his darkness, becomes Keyblade Master, and Kairi requests further training to which Yen Sid designated her and Lea to Merlin. Sora then comes back to The Mysterious Tower and journeys with Donald and Goofy.
Of course, this is just the skeleton idea of an alternate Dream Drop Distance, but boy would it have characterised Kairi.