We can't explain the universe, just describe it; and we don't know whether our theories are true, we just know they're not wrong. >Harald Lesch
If there's just 2 factions fighting each other you could let one of the players control the computer enemy.
I'd rather not do that, since most of the time, one player or the other has a massive disadvantage. Protoss mission 1, for example, the Zerg player could mow over the Protoss easily, early on, or kill that small force.
You could leave all the preplaced units under computer control and give the player just a hatch and deny him the option to expand.
Player 1 brings at least 1 Drone to 'base border'
Move all Drones at base border to 'start'
Move all Overlord at base border to 'start'
Display "No Expansion for you!"
If necessary make existing mineral spots pretty dried out, so he can only build a small force and has to make the best of it. I would even balance it so that he's bound to lose, but for him to win he must stay alive for x minutes.
Or give one player the expansion as soon as the first player builds it.
Maybe, but on some maps, it could hit a resource problem, and one player might wait ages to get his building.
It's just another option. Use it where appropriate. And balance it the way that he can only win if he expands quickly.
Even if there's just 1 player you could split it up into builder, infantry and mech units much like "Verlorene Technologie", a german map. Maybe you know the english version "Lost Tech" or it's PvP variant "Lost Battle".
Haven't tried that map, but if I understand correctly, how would the supply work, if one player controls the units, and one builds them. That'd be an interesting possibility, though.
The builder controls Command center, 1 Barracks, 1 Starport, the medics, the ghosts (Nuke!), the vessels, turrets. Depots are equally split up between the factions.
Once he builds a factory, or the 2nd Starport those go to the Mech Player, once he builds the 2nd Barracks or a bunker those go to the infantry player.
The mech and infantry player then build their own units, and are given necessary tech as needed. For example if the builder builds an academy,
Give 1 academy at 'Preplaced' owned by P12 to the infantry player.
For Zerg just give the units, maybe divide between ground, air and builder.
For Protoss it's a bit difficult with the pylons if you want it perfect. But you could keep it simple and just center a location on a warpgate when it's built and give 1 pylon at this location from builder to infantry player. Chances are this will disable one of the builders cannons or Robotic facilities, but then he would just have to build another pylon and all is well. About the army factions' supply you could again give preplaced pylons to them to avoid disabled buildings.
You could split up between melee and spellcaster to the templar caste and ranged units to the warrior caste.
These are just rough sketches, but I'm sure you can work out the details. It would definitely add fun due to the necessary teamplay involved.
Edit: Oh, what's everyone's stance on the sounds? The only way for them to work is to import the actual sounds into the map, but those add at least a meg to each map.
Wasn't there a way to also reference to sounds from the cd? I think you have to import another mpq to ScmDraft or something. Sorry that I can't be more specific, I just read it somewhere, but forgot about it. Never done it myself.