You will find that your child will choose his or her own names for granparents.
Our first grandaughter christened me 'Grandad Glasses' because I wear them, and my wife is 'Nana Mum' because she heard her mum say "Mum" and added it on. My wife's parents are 'Big Nana and Big Grandad',
When our grandson arrived, I became Pandy - his mum is my stepdaughter, so rather than Dad, she calls me Andy, which he couldn't pronouce, so Pandy became my name.
Other grandparents - my daugher's ex-husband's dad, and her partner's mum are Grandad Jef and Nana Margaret, so these things just evolve naturally - it's more fun that way.