I don't think that Starcraft II will stray too far from the linear plot system of SC and SC:BW. In Warcraft III, Blizzard had to introduce 3 or four heroes per race because the heroes were used in multiplayer, and this is what really cluttered the WC III plot. Since heroes will not be available in multiplayer (barring UMS, of course), I think it unlikely that Blizzard would clutter the plot with a bunch of super-power heroes in the campaign.