This was inspired by a topic at MLP Forums where someone asked if it's possible for cutie marks to change/evolve within the show. It was greatly discussed and I became intrigued by it. [link]
I think that since CMs are obtained by some way of purpose-discovering magic, then they are capable of evolving. Not changing completely, but gaining additions once a pony transcends their basic destiny and acquires great skills related to their purpose.
My OC Wingnut has an evolving cutie mark.
tl:dr - why the hell not?
Anyways here's what I would interpret as Pinkie Pie's "evolved" cutie mark. Three presents attached the end of the balloon strings kinda like weights or anchors. It means that her smile and laughter are a gift to her friends and are so infectious and wholesome, that they bring great soothing and stability to her friends' minds and souls (eh? eh? the weight/anchor thing?)
original cutie mark vector by [link]