In addition to Nashi herself, Makarov could be a central figure in the fanfiction. As the guild master, he has a lot of wisdom to pass on to the younger generation. He might tell baby Nashi stories about the history of Fairy Tail. Mirajane could also be important. She could transform into her Satan Soul form in a very gentle and non - threatening way just to amuse Nashi. And then there's Elfman, who could be teaching Nashi about being strong and brave in his own unique way.
Happy could play a big role. He could be like a little guardian or playmate for baby Nashi. He could fly her around the guild hall, showing her all the interesting things. Erza would also be a significant character. She could be the one who enforces discipline in a very kind - hearted way, teaching Nashi the values of the Fairy Tail guild from a very young age. And of course, Wendy could be there to use her healing magic if Nashi ever gets a little boo - boo.
Natsu and Lucy would definitely be important characters. As Nashi's parents, they would be constantly looking out for her. Natsu could be teaching her little things about fire magic in a very gentle way, and Lucy would be there to comfort her when she's scared. Gray could also be a main character, he might be the one who makes little ice sculptures to amuse baby Nashi.