An idea I had was for an entirely new item. Perhaps a hand held radio or something that you can either activate in your inventory or create a new hotkey for.
When you use it you call for help/reinforcements. Then if their are any Stalkers in your faction on your current map there is a chance (maybe X percent) some of them will move to your position (or even better, where your mouse is pointed). This would greatly improve faction wars. I'm guessing this would be quite hard to implement however.