Maybe a we could have a spy GP. Once used, you can select one of the enemy buildings where units are built, such as longhouses, docks, stables, etc. Then, the next time that enemy makes a unit there, you can see its line of sight. He still controls and sees it like normal. This one is the simplest of my ideas.
This ones more confusing. If you get confused, its probably because I have no idea what Im talking about either. This one is for an actual spy unit. It would be expensive, and maybe have a limit to how many you could build. When you make this spy unit, it would appear like one the enemies units and colors. Example, you're against a green Norse player. You make a spy, which looks like a green Ulfsark. This time though, you can control the spy unit, the enemy cannot. Your wondering now, probably, how your enemy is supposed to recognise the spy. You know how when you move your cursor over an enemy, it brings up the sword icon allowing you to attack it? If the enemy moves his cursor over the spy, he can attack it. If he selects, it will also be named spy. Actually this idea is simple, you may have to read it a few times though.
Lol, typing these ones, I forgot my other ideas. I put them up if I remmebr them
DK