Cover, a place to stay while your shields are down OR places of shooting advantage. Your choice on which on of the definitions of cover are stressed in a map. Bunkers offer shooting advantages while a crate offers well cover for your shields to recharge. I recommend adding bunker like objects or cover you can shoot over AND crouch if you need to re-charge shields. One of my favorite bunkers is simple, make a single open box, geomerge it once with doors, and then add a short barrier in front of it. It actually works well. For even better cover, put a door on its side in the middle of a passageway and add a door to see through the tiniest crack of it. EDIT: all of this talk wants to make me make a realistic, cover map like he is mentioning. Maybe he is on and I can forge with him to get a decent map out of this.
It sounds like you're describing GoW (Gears, not God, of War). Personally, I hate Gears. When I played it, it seemed like one-shot kill, pretend cover, and one life for EXTRA "FUN"! If you fixed it so that it was not as punishing as that, I might play a "realistic" map. That doesn't change the fact that Halo was made for a more action-oriented rather than realistic-oriented game experience. Sure, you can place things that could be considered "cover" but they aren't designed to be. Look at the barrier or window panel. When you crouch, your head is still exposed, leaving you open for fire even though you can't shoot. Anyways, good luck. This has some potential, I just hope it works out.