Ok, I was thinking something along the lines of a siege type of level. One side starts with a base which they have to defend/expand. The other side gets reinforcement periodically with which to destroy said base. To make it really fun, the attackers would occacionally get convecs with which to start their own base (but not till later in the game when the defenders have built up a little). Now, the attackers would have a disadvantage when building their own base since it's not till later in the game, but they do get reinforcements to either attack or defend the new base with. (Well, maybe they don't get combat vehicles when they get the convecs but they could keep some in reserve to protect the base when they get the convecs.)
This sort of level would likely need to be played on a full sized map, so the defender can't watch the whole thing. That way they don't know when or where the attacks will come from, or where an enemy base will suddenly pop up. Also, day and night will be important. The attacker could get convecs with the headlights off so the base won't be visible until the buildings begin coming online.
As for points where the attacker can enter the map, there can either be a specific number of points (probably have markers on the map) or just anywhere along the edge (less likely for a large map).
The location of reinforcements will likely be random (at least for starters). Maybe we can add some way to select where reinforcements will come from. (Hack the shell maybe? Or have a scout in a secure area move to the corresponding beacon?)
As for victory or failure, I see a few possible modes of play evident.
1) Last One Standing Siege.
Either the defenders base gets destroyed, or the attacker is left with no more places with which they can enter the map with reinforcements and all their units are destroyed. The attacker will not be allowed to enter the map at a defended area (either defenders combat units or guard posts, or some other special building).
2) Startship Siege.
Either the defenders base gets destroyed, or the defenders launch a spaceship.
3) Beachhead Siege.
The defender must stop the attacker from establishing so many operational buildings. The number and type of each building will be specified, so no building a bunch of agridomes to win.
4) Resource Siege.
The Attacker (or Defender?) must extract so many resources from the map before being stopped.
5) (One Sided) Capture the Flag Siege.
The Attacker must take and hold a certain number of key objectives before being stopped.
6) Hot Potatoe? Siege.
One side must kill a certain unit from the other side. Say, a cargo truck carrying wreckage/space module. If the defender has the truck, they might have multiple bases to hide it in. (Heh, Garages maybe?) The objective might be to hold out for a certain amount of time, or launch the wreckage/module into space. If the attacker has the truck, it can appear with reinforcements and must be escorted to some other point on the map (and exit the map), or abandoned to make it's own way. False trucks may be added for a bit of fun.
7) EMP Tag Siege.
The Defender might be surrounded by various attacker fortresses. They must try to exit the map with a cargo truck, or get it to another base to launch the cargo at a spaceport. The Defenders must "tag" the cargo truck with EMP and spiders and move it to their own specified waypoint, or exit the map with it. Special rules may be in effect in case the cargo truck is destroyed.
Also, I'm sure you can think of some non Siege variants of some of the above. As for the specific victory/failure conditions, some of the above will need a little more thinking to work out all the details.
