Obviously, Starlight would most likely become the element of Magic, as in the show, that is suggested.
I think the element of loyalty could, possibly, go to Scootaloo.
Genorosity for Sweetie Belle.
And Apple Bloom the element of honesty (Which was vaguely hinted at in Where the Apple Lies)
In other words, they could replace their sisters/ role models.
Of course, the obvious option for laughter is Cheese Sandwich, but I don't think the role would work for him.
And, finally, Kindness. Everyone in the Equestria is pretty kind.
The other possibilities, the Background 6, and the current fillies.