I think a thermal imaging device would be pretty cool. This device would allow you to see your enemies and teammates. It would of cousre differentiate between the two by showing if they were a T or a CT and would cost 3 credits and of course would not damage the players.
Game play with this device would still be challenging because it does not damage the player it only lets you see where they are. A thermal imaging device does not show walls either, so if you go into one room while using this device they may very well be in the next room over. It also gives the element of surprise and still gives the other player a fair chance of surviving.
The tactical advantage of this device is to see where your opponent is as well as your teammates. Gives you the element of surprise as well as the the upper hand in the fire fight. It is especially useful when there is only one opponent left and is either hiding or looking for you on the other side of the map.