OTL, the brother of Henry III, Richard of Cornwall, was elected by some of the princes as German King in 1257, but his chief contender for the title of Emperor was Alfonso X of Castile . So, the challenge is to get Richard be the UNDISPUTED king of Germany, and get to the pope for his coronation as emperor. I figure Alfonso will probably have to die for Richard to have better odds, though (which if its early enough, may see Al's eldest son, Fernando (founder of the de la Cerda line OTL) succeed him.
Bonus points if you can get Richard's son to be elected as emperor after his dads death.