There has been a feature several of my group and I have been hoping BIS would implement into the terrain editor on ARMA 3. One of which is the ability to create cave systems. Being able to have cave warfare would be really epic. Not to mention it would change warfare dynamics quite a lot. The other feature is working rivers and waterfalls/rapids. This should be possible. I have seen some ArmA 2 maps with rivers but they were usually sad representations of rivers. Adding in the ability to have water "spawning" from a spring or cave, (cave systems with water.....) flow down a path and off a cliff side or precipice into a stream below would be really nice. I can see a waterfall with a cave system hidden behind it. Independents and other forces have made camp in the cave system where sporadic skirmishes are waged.
Underwater caves for subs and divers.......underwater backdoor entrance to cave systems for spec ops to clear the caves of insurgents.