Hey man, thanks! Those are some great ideas. You really thought about it.
I like 2.b the best. I added that ice thingking that the poison gas would get trapped in the ice and the enemy army was being mutated somewhere off-screen. I think the chopper should have to go and destroy the ice that has trapped the toxic gases. Maybe there's an enemy lab inside the blocks of ice. Did you see my ice models?
Also, I want the arctic critters to become mutated and have some bosses.
Big question though, is this... How can I make the Chopper a ground unit when it lands. I want the chopper to be vulnerable to attack when refueling.
Also, I think I need to make level 3 have some in-game restart function. So, respawn base - respawn tanks - respawn everything that might die.