Some ideas:
Code:
- a random scroller mode would be nice. with increasing difficulty and a local highscore
- ingame guis (an inventory, ship tweaking and shops)
- cooperative scroller mode (on the same screen or over network)
- savegames (load and save the game status)
- a working and challenging multiplayer mode
- a simple level editor
- motion blur and drug effects
- mirror textures

- stardust
- bump mapping and dynamic shadows

Content:
- we should should make at least one new level and advance the story.
- a multiplayer level
- a under water level (waterplants, whales, sharks, ship wrecks...

- a level on a planet maybe with vehicles, buildings, plants... (collision?)
- male and female player models with animations

Other:
- installers and packages
- webpage redesign and a bug-tracking-system
to be continued...