This would be really cool .

V.1

When a player passes near it , it shoots sometype of bullet .

v.2

Continuosly shoots in a defined direction .

v.3

Continuosly shoots in a defined direction when a player passes near it .

^_^ For me , its pretty cool.

d-day :P
urban combat :P

for me , i repeat, will be really cool !
You can make something like "monster" battles !

suppretion fire :D
when you touch a certain poly, machine guns would open fire, like a prison map :P
or really high, or far away, when you capture a bunker an airstike would happen, which would go great with the destructible/moving polys :P
boats, transporting people :P

EDIT: and landmines :D