Kairi would benefit from a potential Xehanort saga adaptation. You can fix her problems via hindsight and make her journey more memorable.
Even when I was a bigger fan of KH's story, Kairi just came across as an afterthought. So I felt like everyone's hopes for her in 3 were too unrealistic.
She could benefit from a 13-2 style game with her and Sora as the focus but they are the weakest characters in the cast so I don't know if it would work out. Granted Serah improved as a character when she started hanging out with Noel, so maybe Kairi just needs some new friends?