Height-mapped terrain, various static terrain features we can interact with.
Vegetation, walls, buildings. Anything you can hit will be derived from Enemy.
Divided into grids; the world can be huge but nothing is checked outside the current nearest grids. Units move around from grid to grid, or can be bounded, depending
Player:
Singleton - easier interaction with world and enemies
Two (or more) part model, base plus turret
Independent drive and aim control
Munitions:
Lasers, fast firing, no splash
Missiles, wider area of effect, more dodge time
Bombs
Homing weapons
Camera:
Singleton
Two modes
Follow mode for driving
First-person aim mode for better shooting
Enemies:
One base class Enemy; subdivided into Stationary and Mobile, going down from there
Derivations for different enemies, different difficulty and firepower