I have a one piece/Naruto challange and this is orginal.
What if nico robin instead of joining Baroque works she escapes to the elemental countries island and joins kohona to hide from the world government.
In her time in kohona she was there seeing the uchiha massacre and adopted sasuke and helps him in his pain. They are both suvivors of massacres. She lost her home island, sasuke lost his clan (you know why).
OR
she adopts naruto because he was suffering from the same way she was when she was little. They were both ignored from their hometowns and have terrible childhoods (the orginal naruto story tells he was ignored, not beaten up by the villagers like in the other fanfiction stories)
you can choose wich she would adopt (or both) and it will be a mother/son relationship.
I havent decided on pairings for robin because she dosen't trust anyone due to the constant betrayals in her life. She can be single or with anyone of your choice. Nothing yuri or any thing related with female on female pairings. it sickin's me and its out of character. She can be friends with anko because both share similar methods in dealing with men (see her crushing franky's balls in the anime).
These are my ideas, the story is up to you.