- Some sense of game progression to give the player more to strive for than just one increasing number would work well to keep players interested longer.
- Levels, waves, phases etc.
- Enemies of differing size, speed, even movement patterns
- Gradually increasing each level/wave etc.
- Makes the player change the way they play
- Gives an increasing challenge as time goes by to keep gameplay fresh
- Preset obstacles to force movement strategy
- different between levels?
- A bigger screen size to account for all this and more
- less cramped gamespace
- could return to being cramped later on to provide a challenge
- Something else for the player to interact with
- Powerups, detonators, etc.
- Not necessarily another action the player can take, but something else they can do with the one they have ("move")
- offers the choice of avoiding the creeps or trying to pick up something useful that won't be there for long