Obviously this is all speculation, since there's no concrete evidence to suggest what the plot or setting might be.
All we know is who may or may not be starring in the film:
Paula Patton (close to a deal)
Travis Fimmel (also close to a deal)
What do you think? Personally, I like to think it should take place around the time of Warcraft, and follow the events of Khadgar and Medivh. Start at the beginning and introduce an audience without the knowledge of the Warcraft mythology with the first details-Sargeras' activites, the First War, the Opening of the Dark Portal.
But I'd like to know what are your ideas for the film's story? Bear in mind confusion for a first-time audience-so Arthas' story, as great as it is, might leave people confused.