Seeking pattern to surround a granny square

[bbgrl]

Hi. I'm kind of new to crocheting. My first project was a blanket for my baby boy. I used 3 skeins of different colored yarn in a continuous granny square pattern. My original plan was to add another color every year on his birthday. He is already outgrowing it, so i've decided to go ahead and add on. I'm looking for a different pattern that i can use to add on to the granny square. It will have to be something that can be added onto as this will be an ongoing project till he is 18 years old (he'll be 2 in june). Thank you in advance for your help,

[RoseRed]

I would check Ravelry for 'Sampler Afghans'. 

 

There are some gorgeous ones with different stitches making up the sections as the ghan grows.