While both Harry Potter and The Lord of the Rings are returning to screens soon, the two biggest fantasy franchises of the 2000s will both need to face the same challenge when their next installments arrive. HBO’s Harry Potter series has the potential to be even bigger than Game of Thrones, thanks to the family-friendly nature of the source material, and the fact that the Harry Potter books were much bigger bestsellers than George RR Martin’s A Song of Ice and Fire saga.