SC1 campaigns and story were actually for the Protoss, Terran and Zerg. Broad, large scale, expanding the Starcraft universe with each new mission. SC2 on the other hand, has been exceptionally narrow. Wings of Liberty tried to disguise this with a plethora of side characters but Heart of the Swarm completely dispensed with any 'ensemble' cast. The trilogy should simply be labeled the Raynor, Kerrigan, (Zeratul?) expansions.
The entire whole of the Terran has been reduced to Mengsk's grunts or Raynor's Roughnecks. Gone is the grandeur of the zerg with cerebrates commanding massive broods and swarming across the entire sector. Instead they're rag-tag scattered cells who occasionally phone home and get orders from Kerrigan on the psychic radio. The Protoss? Who? Oh, you mean the sad excuse for a different enemy type that they inserted as the Tal'Darim. Exact same thing with the hybrids, except the limited missions you had as Protoss dealing with the Hybrids in WoL was the closest either SC2 campaign has gotten to the expectations of classic Starcraft.
What Blizzard needs to do is completely ignore the dumb 'introduce units one by one' mechanic. If there are new units or significant ability changes they can bring them in similar to how the Corsair (Legacy of the Xel'Naga) and Lurker (True Colors) were introduced. The story should be created first, the missions designed around the story, and then any variations orcreative ideasspun within. You know, a top-down approach. Anything that isn't a pyramid block chart of each and every unit in the bottom row, pidgeonholing some cheesy probably insignificant situation as "story" in the row above it and then dart board chosen planet names for the row above that. That's the WoW design, and that is entirely the problem for the lack of ingenuity or the almighty lore. Fuck the power-ups and single player only abilities, give us something meaningful.