Im writing something like that too! Maybe your question will help me out too. ;D Right now Im tossing up various answers. I personally think it would be something elegant, and meaningfu, and depending on how your story works out, it could very well depend on the mother.
Sophie. I like that name, but I really don't know
Grace. Elegance, and happiness, all in one.
Selena-Elegant, pretty, it means moon
It's probably too cliche - but if he had had the chance to have a child I feel he probably would have named her Lily. I never thought there was another woman in his life he cared about as much.
Then again, it might have been too painful for him. Who knows!
I'd say Lily. He dedicated his life to her and she was the most important female in his life. Can't imagine giving his child any other name..but yeah just like jameswilson said, it might have been too painful to him. And actually if he got to the point to have a wife and a child I don't know if he would really want to remember Lily..
But..that's the only female name I can connect with Snape
I love the song "Iris" by the Goo Goo Dolls it really describes him so that maybe an option it isn't Lily but it is a flower...you may also go along with Abigail which is defined as "A Father's Joy" or maybe Anabelle which means "lovable" another thing is that he loved to read so probably something elegant from a novel hope I helped =)
I like Helena, which means "light". It's also sort of a tribute to his mother, since Eileen and Helena are versions of the same name. I guess it depends on how you think Snape feels about his mother...