I have some ideas.
Campaign Mode - You go around the world destroying the bases. The enemy bases grow and grow when you are not destroying it and when you do they just rebuild. And then when your army is strong enough you could take over that city entirely.
Race Mode - You try to destroy the city before the enemy does.
Build-A-City Mode - Build a huge city, then destroy it! :P
Controllable Unit - A special unit that you can control manually with the keyboard.