I have four current co-operative campaign missions, two of which can actually be worked with as a story (the others were just a bunch of crap I threw together, but are fun nonetheless).
First map:
Attack on the Dominion
Image:
[imgwh 640x426]http://i628.photobucket.com/albums/uu2/ ... minion.jpg[/imgwh]
Description: Fenix and Raynor duke it out with the Dominion. Starting with a small task force, you must destroy the enemy bases and establish ones of your own to combat the much larger threat...
Difficulty: Bordering on Insane. Three large Terran bases (one with a large expansion) and outposts placed "around" can be pretty damn tricky... especially when Purple LOVES to Siege Tank rush the Protoss player. Luckily enough, the heroes are overpowered to no end...

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Second map:
Desert Defense
Image:
[imgwh 640x640]http://i628.photobucket.com/albums/uu2/ ... efence.jpg[/imgwh]
Description: A Terran / Protoss alliance has been formed to drive back the Zerg infestation on Korhal. Wipe them out.
Difficulty: Difficult. Zerg repeatedly take back their expansions, and they don't attack often. But when they do, you'd best have some pretty good defense.
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Third map:
Push 'em Back
Image:
[imgwh 640x640]http://i628.photobucket.com/albums/uu2/ ... emBack.jpg[/imgwh]
Description: Protoss invaders threaten the Overmind! Destroy them, lest they use their dark energies to destroy your beloved leader!
Difficulty: While the A.I. scripts are Difficult, I'd say the actual map difficulty is set halfway between Difficult and Insane. The A.I. expands five times if you let it and one favors Arbiters while the other favors Zealot rushes... beware the Zealot rush...
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Fourth map:
Attack on the Confederacy
Image:
[imgwh 640x640]http://i628.photobucket.com/albums/uu2/ ... deracy.jpg[/imgwh]
Description: The Overmind is in it's final stages of incubation! Defend it and wipe out the Confederate scum who aim to destroy your mastermind!
Difficulty: Difficult. You have 3 players, which lowers the difficulty of the scenario considerably, but there are 3 Terran computers as well. Beware.
On the subject of Attack on the Confederacy, I'd like to say that this map is different than what I would normally do. The very fact that there are 3 players is odd enough (when compared to the other maps), but you are also sharing bases.
Leave comments and [constructive] criticism here, as well as any suggestions or questions you may have. Thanks!