I liked the idea of Rainbow Dash and Derpy clearing the skies before Derpy messed up and accidentally set off a lightning bolt that hit town hall. If I recall, that was the initial setup for the "Last Roundup" scene.
So, here's how I picture the scene. I think it's pretty adorable... I imagine that while Dash may not have always liked having Derpy around (since Dash is so easily frustrated), she would take Derpy under her wing (ba-dum shhh) like she did Fluttershy. My Little Pony: Friendship is Magic