Well, I did make one that has little playtesting behind it.
Scenario: New Gettysberg
Mengsk's push to power is at hand. All that needs to be done is release the fury of the zerg swarm upon the Confederate home-planet and power will be his. However, the Protoss have unexpectedly shown up to cleanse any sign of the Zerg. They must be stopped if Mengsk is to achieve his position of power.
Player 1: Terrans (Raynor and Mengsk)
Player 2: Zerg (Overmind)
Player 3: Protoss (Tassadar)
Abaddon Bhekar Ro
Antiga Prime  Dylar IV  Pridewater 
  
 Torus 
In case the galaxy map above doesn't make any sense, Abaddon has z-axis route #1 on one end and is connected to Antiga Prime by the other. Antiga Prime has z-axis route #2 and is connected to Abaddon and Dylar IV. Dylar IV is connected to Antiga Prime, Bhekar Ro, and Pridewater. Bhekar Ro is connected to z-axis route #1 and Dylar IV. Pridewater is connected to Dylar IV, Torus, and z-axis route #2. Torus is connected to Pridewater.
Terrans: start with 5 marines, 2 firebats, 1 medic, 1 siege tank, and a base on Antiga Prime. They also have 5 marines, 2 firebats, 2 medics, 1 ghost Hero (human Kerrigan), and a base on Dylar IV. In addition, start with 9 workers, an air defense module, and the following Technologies: Heal, Cloaking Field, Bunker, Scanner Sweep, and Improved Reactors. Each territory on both starting planets receives 1 guard token.
Overmind: start with 3 zerglings and a base on Abaddon. Also start with 3 zerglings and a base on Bhekar Ro. They also start with 8 workers.
Tassadar: Start with 4 zealots, 2 dragoons, 1 high templar, and a base on Torus. Also start with a stage III Gateway and 8 workers.
None of the players start with transports.
Protoss cannot build Dark Templars or Dark Archons.
Terran player uses unit pieces from both factions, but only 1 player sheet and 1 deck of combat/technology cards.
Kerrigan's Hero ability is being able to play the big numbers off of any combat card (whether they apply to a ghost or not).
The Rules of Survival Game Mode are followed (conquest points are not gained but converted to resources of the player's choice).
The following Event Cards are removed:
Stage I: Emergency Routing, Scouting Party, Technological Advance, Navigation Error, Refinery Bombardment, Versatile Strategy, Preemptive Action, Subtle Planning, Base Construction.
Stage II: Industrial Strategy, Refinery Sabotage, Progressive Strategy, Unexpected Action, Rich Vein, Air Support, Brilliant Engineering, Reinforcements.
Stage III: Stalling Tactic, Accelerated Progress.
Terran player wins when all Protoss bases are destroyed.
Zerg Player wins when all Terran bases are destroyed.
Protoss player wins when all Zerg bases are destroyed or when 2 The End Draws Near cards are played.
The Terrans must be careful not to destroy the Zerg bases, while the Protoss must be careful not to destroy the Terran bases. I doubt the Zerg have to worry about how much they harm the Protoss in the process of destroying the Terrans.
Since this isn't fully platested, some tweaks may need to be made to more balance the difficulty for each player. If the Terrans are too powerful, remove 1 starting unit on each of their starting planets. If the Protoss are too powerful, add in more event cards (but never the z-axis route changers), or make it so that 3 The End Draws Near cards need to be played. If the Zerg are too powerful, well, I'll leave it to you on how to fix that.
If none of those work, tweak as you like.