I have seen a video showing raycasted line of sight some time ago. I think, that you really should implement it to game, as well as some lightning system, that gets precalculated at begining of round, and updated when terrain gets modified. If it would be pre-calculated you could even use ray-tracing for reflections and semi-transparent materials. Also some dynamic lights (like a torch) would be nice, especially with dynamic shadows. This could as well have a bit lower resolution than terrain map, and just be blurred to look quite realistic...

And some criticism:
1)Liquids do not look like liquids even a bit... I would suggest to animate them, and make semi-transparent so things behind them are visible (also items in water would be less visible). You also could use some shaders for distorting liquids textures and objects inside it....
2)Said line of sight... I'd really like to see it ingame...
3)Said lightning system... I loved the Vulcano scenario for dark caves, for OC you could do something more accuarte and maybe dynamic...

And some other suggestions:
On IRC I have mentioned heat distribution system, I think that it could be connected with fire mechanism... Things have a temperature above which they start flaming, and things on fire or explosions produce high temperature. Temperature could be a map of resolution 16 times smaller than terrain map, and could be transferred depending on pre-calculated heat resistance for each heat map pixel to nearby pixels. With this explosions could produce a heat wave, that enflames things, this would allow objects on hot maps be set on fire way faster than normally, and chosing place for a base would become more difficult task, as player would have to worry aout temperature. Also this would allow realistic lava->stone transformation or freezing of water on surface(example: You go with barrel of water to surface, it freezes, you go near fire - it unfreezes.). I dont know how is Clonk engine made, but it does not look like too CPU expensive thing, especially if it would be calculated onle every few frames.

Maybe some wind system? But I guess that would be really too much work, and would use too much CPU to be a good idea. However... there is OpenCL, in few years computing power will probably greatly increase with use of it, as GPU or multicore CPU computing will become common practice... this could be a good idea - building windmill on top of hill is really way better idea than building it in a valley, also it would bring some wind to caves...