I had the same dilema when our grandson Logan was born. Yep, I said grandson! My dh has a 27yo son from his previous marriage. I always say that I can't take full credit for our son, but I'm taking as much credit as I can for our grandson.

DH and I have been married 16 years and are now finally at the ivf stage. (He's 51, I'm 41)
I tried to come up with something cute and different since Logan has 4 grandmas, (mom's mom and step-mom, dad's mom and step-mom), but nothing ever stuck. Anyway, I went with whatever they wanted to call me at first, my only stipulation was that they didn't call me Grandma Kristy. I like my first name, but not to have my grandson call me by it. When he was a year old they were visiting us in Texas for a couple of weeks and had gone to run an errand. When they were pulling back into our driveway Logan got all excited, like he recognized where they were, and he started saying Ga Ga over and over. Anway, that's what I've been called ever since. I love the fact that he gave me my name.
Don't know if this helps at all, but it's definately a different twist on grandma.