Heres some crap for you to make:
2 Teams, they start off by placing (Insert
Arbitrary Number Here) Combine Turrets around their respective bases to protect (Insert
Arbitrary item here) The opposite teams does the same.
Game starts, each player has "Resources" and can gain more by killing enemy npcs. The resources can be used to buy more defenses or buy attacking npcs such as combine soldiers or zombies. The npcs of the same team do not attack each other, but attack the npcs of the opposite team only.
The players' jobs are to guide your forces around and break the enemy barricades to kill/destroy (INSERT
ARBITRARY ITEM HERE) using mouse drags and stuff. Every player has their own resources so they can buy whatever NPC they think is necessary. Whoever kills the enemy (INSERT
ARBITRARY ITEM HERE) First is the winner.