How should it be done?
Do you think they'll do it the same as Breaking Dawn and The Hobbit and spread the story across several films or will they try to focus on just the main storyline?
For WCII, I think they will have to split it into at least two movies. The first would focus on the ongoing war between The Grand Alliance and The Horde, ending with Gul'dan abandoning the Horde to raise the Tomb of Sargaras. For the tie-in, they'd have to do a flashback scene showing Aegwynn "beating Sargeras" and burying his remains. Unbeknownst to her of course, Sargeras set the whole thing up to pave the way for Medivh.
The second movie would cover the events of Beyond the Dark Portal. Khadgar, Turalyon and Alleria would have major roles of course. They would also cover the collapse of the Horde, the Orcs' internment and Thrall's escape. Then there'd be a tease of WCIII's film with Medivh visiting Thrall in a dream.
As for WCIII, I can't see them covering everything that needs to be covered in one film. They'd need at least two to do but 3 would allow them to go in slow and even get away with doing certain events out of order a bit. In other words, each movie would focus on one of the major events that happened at the time: The birth of the Scourge (and the Forsaken), The Orcs' Redemption and the end of the Night Elves' Immortality.
WCIII Part One would focus on the Scourge though events of the other events would be losely referenced. They have plenty of reference material to not totally screw it up. The movie could even open with Kel'Thuzad meeting Ner'zul in Northrend. It would then go through the events of Christie Golden's Arthas: Rise of the Lich King (minus the horse obsession hopefully) from Arthas' battles with Mal'Ganis to his legendary duel with Illidan. After he dons the Helm of Domination, post-credits you see Thrall and his forces making landfall on Kalimdor.
WCIII Part Two would focus on the Orcs severing their ties to the Legion, Grom's death and their blood pact with the Tauren. This movie's focus would be to show the Orcs regaining control of their own destiny and no longer being slaves to their past. I expect Saurfang, Eitrigg and possibly Broxigar to make appearances. The movie would end with Illidan being released by Tyrande. Desparate times!
WCIII would be the big finale: The movie would open with a summary of the War of the Ancients while also referencing events from the first two movies that happened. It would then open with Jaina and her people landing on Kalimdor after cutting her ties with Arthas. The climax of the film would of course be Jaina, Tyrande, Malfurion and Thrall being set up by Medivh to meet each other. After the Battle of Mt. Hyjal, the Orcs begin contruction of Orgrimmar on Durotar--their first permanent settlement on Azaroth. The movie would end with word getting back to Jaina and Sylvanas about Arthas becoming the new Lich King. Post-credits, they could show additional scenes in which The Forsaken join The Horde and the Night Elves join The Alliance.
...This opens the door to making movie versions of events from Vanilla WoW =D
IF they were to go that route...well, separate post XD