Yet another Dothraki character who bit the bullet fairly early in the series was Mago, a member of Drogo's khalasar who becomes an adversary of Daenerys when she prevents him from raping Eroeh, a member of a conquered community. Refusing to accept her authority, he insults her, which results in his death at the hands of her husband.
In the novels, Mago neglects to challenge Dany's authority until after Drogo's passing, where he rapes Eroeh before passing her on to his new Khal, Jhoqo, and several other members of his new khalasar before they cut her throat. This leads to Dany swearing a powerful oath of vengeance against the pair.
Not a great deal of attention was paid to Mago's on screen death until George R.R. Martin publically questioned it, spoiling the fact that Mago would have a part to play in The Winds Of Winter and/or A Dream Of Spring. Dany does indeed encounter Jhoqo's khalasar at the end of A Dance With Dragons and will therefore seemingly avenge Eroeh at some point down the line, but what will Mago be involved in before then to prompt Martin's questioning?
The simple on-screen remedy will be to invent a new character, or simply merge Mago's role with that of Jhoqo, who will be cast for season six and can easily be depicted as cruel and barbaric to ensure Dany's animosity.