Impossible Scenarios is a famous style of gameplay created in Starcraft.
Each player has some melee units and all players need to destroy a large number of computer enemy units.
Thanks for the ideas, I'll consider them when making a 3rd Pool.
I made an Impos also in Warcraft III, with a Custom mode, allowing the host to choose the units of the AI, and for each player.
You can get my Warcraft III maps there, if you are interested :
For the Overseers, there is a trigger that forces every unit which exits the battleground to come back in the center, periodically.
Well, in some Starcraft 1 Impossible Scenarios, I saw modified units in conjunction with melee units. Hopefully that might give you some inspiration.
Idea One: Army Support
In one very interesting scenario, I saw the players acting as a support squad for a larger AI Army attacking an enemy AI army. The Allied AI Army was made up of Wraiths and mobile Siege Tanks against layers of Terran Defenses (Bunkers with Marines and Siege Mode Siege Tanks among other things). The players got various units, Starcraft 1 Queen, High Templars, Battlecruisers. And the player was forced to use abilities to help keep their army alive (As Queen, using the Broodling Ability to insta-kill Siege Tanks. As High Templars using Hallucinations to soak up damage etc.).
Idea Two: Divide and Conquer
Instead of all the players' forces being amassed at the beginning, how about them being split up and being forced to all push towards a common area to regroup, and from there finishing off the mission?
Idea Three: Base Building
Player is given a start up base with a very limited amount of resources nearby, and must destroy the enemy base (possibly using only basic units, should also probably disable Medics if player is Terran.) (Multiple enemy bases in succession? Each with a small amount of resources for the player to use after clearing out.)
Idea Four: Stealth Assassination
Aim to kill a single unit out of a fortified base with the use of Ghost(s), possibly add Energy Regen Packs or Stations at key checkpoints so the player doesn't have to wait extended periods if they screw up slightly with Cloak.
Idea Three: Mix and Match
Being able to pick what troops you want to take into the battle, I know at least one Warcraft 3 version allowed you to do this. It actually made the team work together and figure out what everyone was gonna bring in.
Note One: Detectors
You should somehow close off each stage, because sometimes when attacking a detector, it'll run away right off the stage and onto another stage, making finishing the stage impossible.
Eh 'impossible scenario' is the type of gameplay, even if it's easy ^^
For the moment, it's hard to see if it's too easy as it's simplier if you can control all units, maybe as multiplayer it's harder, I'll need to test, first levels are rushable, but others you need strategy or you lose, if you have replay I'm interested to see how people play it, maybe I can correct some imbalances.
I have the impression that it's harder to make interesting impos with SC2 units compared to WC3 (I made WC3 impos too) or SC1, maybe I need more knowledge about units, I need more ideas for interesting impos, I'll test your impos ;)