The only trilogy I'd be cool with is sort of an 'end war' trilogy, by which we fight the big three factions behind WoW: Titans, Old Gods, Burning Legion.
Part I - Return of the Pantheon: The Titans return to Azeroth, and believe that the planet is beyond saving. We, along with the aid of Ra-Den and Tyr, take the fight to the Titans to prove that we are capable of fighting the wars ahead. The expansion could be based on some sort of huge continent suspended in space, that acts as the base of the Titans. Think something like Ulduar crossed with Outland.
Part II - Revenge of the Black Empire: Having proven ourselves to the Titans, they aid us in ridding Azeroth of the Old Gods, stopping the reformation of the 'Black Empire'. N'zoth plans to free Yogg Saron and resurrect C'thun, and we must fight the forces of the Old Gods, revisiting Ulduar, Ahn'Qiraj and the islands around the South Seas, as well as the city of Ny'alotha at the bottom of the Great Sea, where N'zoth resides.
Part III - The Fallen Titan: Having defeated the Old Gods, the citizens of Azeroth with the aid of the Titans, take the fight to Argus to defeat the Burning Legion and Sargeras himself. This would be the penultimate expansion.
That's the sort of trilogy I could get behind. They'd feel unique enough but still connected.