The cover and terrain for the open fields and hills is great but I think in urban environments with more buildings it looks a little bare...

If there were benches or broken down vehicles, a market place, even half destroyed buildings to fight through? The levels of any shooter, from any camera angle, are what make the game come to life! Giving urban environments landmarks like a fountain or roundabout, a square with statues etc Just to give the urban areas more life? I'd do it myself if I knew how to A. Create a map and import 3D objects into it and B. get the AI to recognize it as cover.

Oh and movement wise I'd love to see a dive to prone and a quick roll for avoiding danger!
Gun fire doesn't seem controllable either, would be nice if it got more erratic on full auto and more accurate with bursts!

I'm not sure how much more intensive that would be for processing but...

The new map that I'm working with currently will eventually have at least some of the things you mentioned.

There'll be chairs and tables to decorate a few outdoor restaurants, shops will have stuff in front the entrance that should make a nice cover, you'll be able to climb ladders to access the roofs, use pipes for cover, etc. There will also be a bunch of new things that will not be significant in terms of gameplay, they'll just decorate, create variety and atmosphere.

While I like the idea of half destroyed buildings, I haven't given much thought on how I would implement them yet. My ultimate vision for the game actually includes completely destructible buildings that would naturally allow any building to become half destroyed also, but like I said, I haven't thought it through yet. Creating static half destroyed models with proper bullet collision detection might end up wasting precious development time if then going to destructible environment requires a completely different approach.

Importing 3d models into the game isn't currently possible, but support for it will likely come in the future. Map modding is already possible to some extent by messing around with media\maps\map1\objects.svg with e.g. Inkscape. A lot of things there will change for beta 0.5 once again, hence I wouldn't recommend starting a new map mod right now too seriously.

While it's not exactly a dive to prone movement, I'm going to do something to allow the soldier to be pushed away by an explosion on the outer range and fall down, so that the soldier will stay alive but recover from the impact for a while until getting back to action.

Firing is supposed to have a somewhat clear accuracy penalty if shot by holding the action; maybe the parameters aren't quite right. Despite my intentions, I still haven't made the dynamic crosshair that would also visualize the current accuracy factor. I might also bring out the possibility to control the firing mode, single/burst/auto, there are places where it could be handy.

I've also had problems with walls that are too high. My understanding of a trench is to offer cover while you shoot over the top. Sometimes I find the trenches are too high to shoot over the top. This doesn't make sense to me and makes you vaunerable to attack from any high side.

Actually trenches are not always intended as fighting positions. Often a trench is primarily meant as a safe passage to and from proper fighting positions. Ad hoc fighting positions are sometimes built into the trench as well, as an afterthought.

But I suppose it might be more fun if the player was able to fire out of the trench (a lot) better, although it might often make attacking unnecessarily difficult.

For now, the trenches have been made mostly to provide cover where you can run straight up and change position, like Jason9mm said. The sandbag walls here and there that work the other way around, providing a crouching cover to shoot over. I agree some of the trench walls could be made lower, but just for now, they've been all made for same primary purpose.

Also, it encourages having fights inside the trench, which is likely more exhilarating than shooting enemies in the open. You can usually defend yourself from a single enemy approaching a high trench wall by going close to the wall on the low side near the position where the enemy would jump down to the trench, so that he won't be able to shoot you, and by stabbing him with knife in mid-air when he jumps

Nice suggestions, especially the movement options. A slide into crouching position like in Ghost Recon Advanced Warfighter when running and going into crouching position would also be awesome (especially with a little dust cloud when doing this).
But there should be a difference between running and slow walking in order to introduce that (maybe Shift to run, or toggleable running), which would also make sense in regards to accuracy, IE when you fire while running your accuracy drops, when you walk you have greater accuracy.
It's cool that Pasik is trying to add some unique objects into the urban map like you suggested.