1. Rhaegar Targaryen
One of the most important characters in the literary series is Rhaegar Targaryen, yet despite this the part has never been cast and the man himself has been mentioned only sparingly. This is likely due to the fact that he died seventeen years before the shows first season. Given that Daenerys (Rhaegars sister, though he died before she was born) will have little to do in the upcoming season (assuming they do not advance her storyline from A Dance With Dragons too much), it would make sense for Barristan Selmy, a man who fought with Rhaegar on several occasions, to tell her stories of her brother, exposing viewers to the events that led to Robert Baratheon rebelling against Targaryen rule and taking the throne for himself, such as Rhaegars kidnapping of Lyanna Stark and the epic Battle Of The Trident (which would allow for cameos from Sean Bean and Robert Addy should producers be able to remove age and weight from them digitally. Such scenes could also be used to introduce the character of Jon Connington (who becomes important during the events of A Dance With Dragons) to television viewers, whilst also illustrating the motivation for actions that new character Oberyn Martell will take and introducing the prophecy of the Prince That Was Promised, one of many important prophecies, visions and dreams that the series has yet to explore in much detail. If theres one thing that Game of Thrones has been lacking, its backstory, and Rhaegar is the perfect figure to orientate the occasional exploration of recent Westeros history around.