Duo is a relatively simple, arcade-style 2d shooter in which one horizontal line runs across a screen thinner than it is wide. Along the line runs a circle with a small "gun" protruding from the top end; the circle is controlled with the arrow keys or the WASD keys, but the definitive feature of Duo is the ability to rotate the "gun" to either side of the line. Targets, such as bubbles or aliens, fall from either side of the screen toward the line (which acts as the center of gravity), and the player must continually gather firepower upgrades from kills and switch between sides to avoid taking damage. The highest-level upgrade creates another gun (sans the first gun's powerups) on the opposite side of the circle. Once both guns are at full capacity, the need to rotate disappears, but the amount of obstacles increases. There would be a plethora of levels, from having one ship on the bottom of the screen (and thus, one side to manage) to a hailstorm of bubbles and a moving line.

That sounds pretty interesting. :)

Personally I wouldn't add a second gun, or at least not allow it to become as strong as the primary, as the rotating might be just what sets your game apart from other space invader games. Not that it's a bad thing though.
Moving the line sounds good, but how about an enemy type that attracts the line by magnetic force or such? Creating a curved line, which swings back into place once that enemy is destroyed.

Ow, and why just a line? Why not a circle, or rectangle... or multiple lines, that slowly connect to each other as you progress through a level... :)

Sounds like a good idea, definitely.