The spire, and the mutalisk and brood lord strains, are exclusive to Kerrigan in Co-op Missions. The latter still requires a greater spire mutation. Additional upgrades for the two strains become available when Kerrigan reaches Level 11.
The spire was a simple update of the original game's sprite artwork. The challenging aspect of creating the spire in 3D was conveying the height of the building. StarCraft II has height limitations for building models, especially for multiplayer artwork due to the altitude flying units move at. Buildings exceeding certain heights either require air pathing blockers so flying units don't clip through them, or height adjustments so they're lowered under the flight elevation. Air pathing blockers are not an option for multiplayer art, so the spire was lowered for the sake of functionality. Another consideration was making this building somewhat humble so the presence of a greater spire would be more significant.