I'd tend to focus more on the psychological and emotion affects rather than physical properties of a field.
-Booby traps (not even set up by a team, but put there by the organizers/field operators)
-Foreign propaganda (pamphlets scattered about, rousing speeches or music/national anthems on speakers within a fortification, "Join your German brothers and you will not be harmed," general demoralizing of enemy forces, etc)
-Overhead explosions (sound shells)
-Live fire (hopefully people can use common sense about it)...(live fire as in combustion, not shooting live rounds)
-The smell of blood/fake blood (war torn countries can be all that torn unless there is blood splattered on a wall or two, nothing creepy than waltzing into an empty village where the air is heavy with the smell of blood)
-the sounds of fire-fights and artillery being heard from places out side the bounds of the field
A lot of cool physical things can be added to, but for myself, the full effect comes from the emotional state one ends up in while playing. If I'm playing milsim, I want to feel like my life cold be taken at every turn.
But yeah, underground tunnels are an idea of thought of as well, especially if they appear shoddy and you feel like they could collapse at any moment...obviously should be built well, but with the appearance of poor/hasty construction.
As for unrealistic, how about a field like a temple/dungeon from Legend of Zelda? Team A attempts to traverse the dungeon, solving the puzzles and dodging the traps, while team B acts as the baddies, or both teams try to solve it simultaneously, but only one can get the heart piece/treasure at the end. The latter would lead to fighting over keys, maybe even the teams helping each other some points. Maybe set it up so the teams must help each other at one point.
Out of Sport. Have Fun!