well it would be great if he came back, he played him reallly well for two scenes in CoS!
There is just one problem Ralph has fallen in love with his character! I do not know if he would let Chirstian play him again!

I have heard Ralph wants more scenes in book six which could be bad for Chirstian!
I hope Chirstain does come back to the role because it would be so cool for the fans and for the movies. It just depends upon Ralph!