There is something like Spy in the Killzone games and it was very balanced. As soon as you attacked or were shot, you lose your disguise and then you had to wait a certain amount of time, something like 15 seconds to redisguise yourself.
"he was aiming down an x-ray ACOG scope. This enabled him to see enemy troops hiding behind cover, and gave him the option of firing through surfaces to kill them. It was a rather helpful tool in a chaotic and tricky situation.
According to recent rumours, the x-ray ACOG scope is available in multiplayer, but only as a killstreak reward. Allegedly, players can activate the killstreak and attach the x-ray scope to whatever gun they are using for a limited time only."
That + Deep Impact... Prolly gonna be way overpowered.
Wilson's description of Spy simply puts you on the enemy mini map as a green triangle, not makes your name green when your opponents see you (which would be way too much - it would ruin hardcore, for instance). Even if it is just the green arrow though, it would still be a horrible implementation. It wouldn't really affect bad-mediocre players, as it takes a lot of experience to really be able to fully utilize your mini map.
It would make Ghost way more powerful (this could be fixed somewhat by making Spy not work while UAV was up, and of course there is no guarantee that Ghost will even be in BO2), and even the potential for somebody using the perk makes the mini map far less effective as an overall game tool by making it much less reliable in terms of identifying traffic patterns, spawns, which flanks are covered, etc. It just seems like a super powerful version of Ghost that really only works against people who have learned to correctly use it through years of practice.