I had the idea, forgot to express it.
Well, even if people evolve, Tsubaki really feels like a big sister to Kousei and Nagi really is a piano student, not a little sister. Writing it like that, I feel like Nagi might have a little more chances than Tsubaki. But that teacher/student relation would probably be limiting Kousei, even if the age difference will become marginal overtime. Unless for some reason Kousei and Nagi live their lives separately for 5 to 10 years and are reunited after that, it's hard to see them together if they share that Prof/Student relationship.

Big sis Tsubaki is out in my mind, because Kousei has shown no love interrest for Tsubaki and was even surprised Tsubaki had such feelings for him. After he learns those feelings, he didn't really show any change that would hint he might be interrested in the near future. From the flasbacks and everything we got from the Tsubaki-Kousei relations, I got the feeling only a big sister love is in Kousei's mind.