I always thought of this as a advanced 2 on 2 game. A lot of it is pipe dreaming, probably. Although I wonder if its possible.
Simply enough, theres four players, the top two make up Team A, and the bottom two make up Team B. Team A is the defending team, Team B is the attacking team.
Create a very, very large map. A world-sized map. There are three bases. Base 1, 2, and 3.
Base 1 belongs to team A, it has a extremely excellent defensive position, however, it has limited clear room to build on. There is also limited resources. Maybe one or two of one-bar rare and common. Also add a geothermal vent or two.
However, at the core of the Base 1, there is an A.I., that uses the color black. It uses the exact same A.I. as the La Corrida AI with modifications. For example, unlike the La Corrida AI, if it feels it has 'Too many' combat units, it will send off half it's force, and transfer it to Player 1. The AI will have access to Three-bar ores. The AI Defending Base 3 also has a phalanx of turrets here. These turrets are in fairly crappy positions, and only serve as a rather artificial first line of defense to prevent rushes and allow Team A enough time to solidify a defensive position.
Base 2 will consist of Team B. They have poor defensive position, however, they have excellent ore. They will also have an A.I. assisting them. Their AI will be plymouth, and mostly focoused on base defense. It will send combat units to Bases 1 and 3 at random intervals. However, if there are enemy units remotely near the base, than it will send all combat units to meet them. It will also build a lot of turrets.
Base 3 will consist of Team A's Main force, They have average defensive position, and good (But not great) ore. It also has an AI, the AI focouses entirely on defending Base 3 from intruders, however. It too is plymouth.
The objective is simple. Destroy the Base 1 AI's Command Center. Team A's objective is to destroy Team B's base. Team A can lose Base 3 and still be able to win.
PS: I posted this to allow people to draw ideas from my original idea.