1. Brush size keyboard shortcuts --as in photoshop, increase and decrease brush size with ] and [ keys. Maybe SHIFT+[ and SHIFT+] to change the height..
2. Rotate camera around a point: so you wouldn't have to walk and turn and walk and turn and rotate etc. Like if you press SPACE and move the mouse, you'd rotate around the center of the view instead of walk through.
3. Import and export heightmap images for terrain. I think this is very important, especially if you are converting previous HMP heightmaps to .icx versions.
4. Import MDL geometry into IceX so user can work with terrains with overhangs and concave surfaces. Geometry could share the same xy proportions as a standard icx terrain. This would take special advantage of the benefits of using MDL for terrains instead of HMP, not to mention being very cool and useful.
5. Opacity slider in paint window.
6. Brush size slider range. I've noticed that with the standard terrain size, it's sort of impractical (for me) to use brush sizes over 5 in radius. So I go from 1-5, usually, but the slider will go WAY up to 100. I recommend making the brush range somehow wider, so there is more variety while working up close than just 1-5.
