My daughter's granddaughter calls her GrandmaDee or GrandmaBear when they are teasing each other. I noticed that your avatar name is "mamagus". How about GrandmaGee. Or as one of our members is called "GeeGee". My great granddaughter calls me Nonnie and she called her other GGmother "GiGi". We loved that. Short for Great Grandma.
I think I voter for GeeGee or GiGi or GG. Love that!
My step grandchildren all call me grandma and their blood grandma is called grandmama. It works for us.
This little tike may find a name for you that no one else has thought of. He/she will have quite a few names to figure out. Make it easy for him/her.