Sheldon from big bang theory had a nice term using me-mar
My nephew called his grandma mima :)
Working at a child care center we hear TONS of names for Grandma's because this happens all the time when they come and pick them up. My top pick would be G-G. My mom is only 45 and she was 40 when my brother had his first...she calls mom Nannie. My grandma (Mom's Mom) wanted to be called Memaw because she was too young to be called "Grandma" A friend of my boyfriends calls her grandma Mum (Granted she raised her). I even knew a kid who called his madolla. like Ma....Dollar because she gave him a dollar each time he saw her. My suggestion would be GG...or let the baby decide when he/she starts talking...trust me you'll try one thing and it'll end up something else!
My mom is nan or nanna. But my nan, which is my daughter's great grandmother, we refer to as GG, short for great grandmother.
My son calls my mom "honey". Lol he also calls his other grandmas "nana" and "nani"
Everyone in my family (my brothers, dad, & myself) started calling my mom grandama, needless to say, she hated it, she is only 16 years older than me. Now my kids are 12 & 8 with cell phones. Grandma has been chopped sown to gma (& gpa for grandpa). It doesn't seem to bother her as much, lol.