What should my son call his Grandma?

I'm due in August with my first baby (my mother's second grandson). I was looking around for names they could call her. She is 49, and feels to young to be called Grandma/Nana. She likes Mom-mom, but I'd rather she's not called that because it's so close to mommy. My nephew is also too young to really be calling her anything, so it's okay to change her "nickname". She was being called Mimi, but she said she doesn't feel like a Mimi. If there's any suggestions you have, I would greatly appreciate it.