Here is an unsorted list of features in case you need inspiration. This list does not contain bugs, cleanup or ongoing improvements to existing features; only new ones. Feel free to use this list, or to do something else altogether. These haven't been fully fleshed out at the moment, they are only ideas.
Weather; rain, snow, hail, wind, dust, etc.
Time of day; morning, night, transitions between them.
Map able to define dust effects, fog, other localised effects (e.g. sparks).
Scriptable camera for storytelling
Campaign
Lua mouse pick, dialogs, custom hud images, etc.
Lua direct gamestate manipulation
Network game registration to a central server to allow for online game discovery.
Trains to run on tracks.
Complex maps with multiple heightfields etc.
Texture and/or bumpmap splatting for heightfields
Bumpmapping for entities
Lots of new entity models and new entity types
Proper animations when shooting; arm pointing in the right direction, etc
1st person improvments; on-screen gun, etc
Gametype able to specify 1x image overlaid over the screen for HUD
Vehicle passengers
Melee weapons with visible differences and animations; chairs, golf clubs, etc
Complex dynamic lighting
Rejig import to support customization, joysticks, gamepads. Possibly use SDL_input
Here is an unsorted list of features in case you need inspiration. This list does not contain bugs, cleanup or ongoing improvements to existing features; only new ones. Feel free to use this list, or to do something else altogether. These haven't been fully fleshed out at the moment, they are only ideas.