Hard to say, but I would guess by the fact that the terrain is set and not even terraformable, and that they are planning to have many different biomes, they are not going to put in a lot of special terrain altering features. They have however said during their architecture section of the design journals that it will be possible to use siege mechanics to start fires, and forest fire were mentioned in that section. Its very possible that along with wild plant life that domesticated crops may be burnable, and if so that would certainly give off an appearance of a razed and brutalized warfare landscape.
As for bodys disappearing, I imagine that there will be natural mechanics for body decay in order to prevent lag from over abundance, namely I imagine monster and animal corpses will decay out fairly quickly just because of game mechanics. Player bodys, and possibly even racial NPCs, will most likely have a longer decay period and special mechanics. In the last Q&A a question was asked about funerals in CoE and it was revealed that almost all cultures will have special practices for disposing of their dead. It has also been shown by the racial characteristics that disease will be a large part of the game, and so I have little doubt that there will be some mechanic implemented with unburied/burned bodies causing disease so as to encourage settlement leaders to act in the spirit of roleplay and dispose of their town's dead folk properly.
The only real exception I can think of is the Dras, who being completely immune to disease can leave dead bodies around their area to ward off would be intruders not immune to the disease. Plus their ability to eat even spoiled meat might mean that the dead can serve as emergency rations. They probably wouldn't do such things with their own people, its already been stated that they have a special funeral right for their own people, but enemies I could see being turned into bloated bog bombs of disease.
All just speculation here of course.