Hi Harry, 

It's a beautiful song. I think "stars' is used metaphorically here to refer to some kind of pre-ordained arrangement by the heavens on how a romance will turn out. 

In the song, the singer laments the fact that her lover sees their hands "are tied" and "it's not in the cards", but decides "who can stop me if I decide that you are my destiny." 

"What if we re-write the stars?" What if both of them decide to take control and re-write their situation, create their own rules and defy what seems to be external forces caging their love for each other? Then "nothing could keep us apart."

This is my interpretation. 

It means to rewrite destiny.

“Written in the stars” is an idiom that means something is meant to be.