I think neither Tsubaki nor Nagi could become a love interrest for Kousei. Should he overcome his budding love for Kaori and become available for another love relationship, it would probably happen outside the character cast we have here. Probably near of post music college graduation. And that would make a hell of a spinoff if carefully written like that show was.
Remaining cast not mentioned are Emi and Nao.
Emi is a mix of a strong oponent and also great Kousei fan. Being both pianist is detrimental. Also there's no hint at anything yet. But they are young teens and life is full of surprise encounters, even more so as they are top tier pianist, they WILL meet again along their musician life.
Nao is the under-underdog. She's behind Tsubaki who was herself behind Kaori. But she's smart and not that far. She also has time for growth. There's absolutely nothing hinting at any kind of relation with Kousei, there's only her screentime in the same show. So I had to mention her.
Then there are all girls at the same school and all the girls he will encounter from now.
The biggest hurdle being able to keep a place for Kaori in his heart, without closing doors to every other girl.