If I can add something, you totally should see Silent Storm (and it's sequels Silent Storm: Sentinels, Hammer&Sickle) and how they handled destructible environments.
If you can make something like that AND expand on that idea (i.e. not just having bullet penetration and wall/floor/ceiling destruction) you will be my heroes forever.
Expanding how exactly?
Well, for example instead of simply disappearing broken pieces of walls and floor, bricks, concrete, etc could be made into physical objects that can interact with other characters. Like instead of shooting very damage resistant enemy you could simply crush him underneath fallen wall by shooting rocket at it/throwing grenade/planting remote explosives beforehand. Or breaking floor under him and cause him to fall to his death.
This dream haunted me very long, because IMHO if you mix X-Com's Geoscape with really deep tactical gameplay of Silent Storm it might be the best Tactical Combat Operations simulator ever.