Producer Chris Sigaty explains that each release will "come with upgrades and changes to the multiplayer." Each expansion will also have a fully fledged single campaign.
The first in the trilogy series then will be Terran: Wings of Liberty, followed by Zerg: Heart of Swarm and finishing with Protoss: Legend of the Void. Each of them is to sport 26-30 missions in the campaign as well as add tangible upgrades to online play.
"Each new single player campaign will come with upgrades and changes to the multiplayer. The main difference here is that instead of giving you a tacked-on extension of the single-player experience as you often find in an RTS expansion, each title comes with a fully realized campaign," reports , having a chin wag with Chris Sigaty.
"Chris explained that each of the three installments would play a bit differently as well, with different meta-game types in each. The initial campaign would feature Terran Jim Raynor, with a meta game that involves upgrading technologies. The second focuses on the Zerg, with a meta-element involving diplomacy. I mentioned Star Control, and Chris said that that sort of gameplay element had been discussed."
How the pricing will work or when we could expect one of the expansions to follow another is all unknown for the time being. Three doses of StarCraft goodness before it's over - excellent.