1.I know a lot of people without 3d modelling skills, use the primitive planes to get more out of the engine, but being limited to planes only is well...limiting!
I suggest adding more primitives like cylinders,spheres and maybe a tube.
A cube would be good too so people dont have to place 6 planes.
2. Add support for loading assets from outside the default folders in the level editor, makes it easier for people using custom assets in the CS and FC/IFC to develop so they dont have to have their assets in 2 places(one in static_objects folder, and one in their mod folder), or do the load another mesh on a model work around.
3. Add support for moving the camera through script to a specified area,(script area or playerstart area fx, you decide) rather then having to script the exact transformations, will help a lot of people out who want to use non-first person viewpoint for "cinematics", mostly intros.
4. Give particles a wireframe box(like boxlight or plane) so you know exacly how much area they cover, specifically I was thinking of the rain particles
5. add an option to make the UI dark grey, because nowadays everything is dark grey