This might be a very tricky thing to implement
, but it won't hurt anyone if I suggest it.
A skill that would allow units to disguise as some other units or even as units of another side. For example thieves from The Settlers 4, changelings from SC2, hero ability in Battle for Middle Earth 2. Here are few examples what I mean:
1. Strong military unit, which would look (to an opponent) as regular worker, luring an attack. Or worker as super-powered unit, to scare enemies off.
2. I would have an unit, that could disguise as an opponent basic military unit and I move it to enemy's base. His own units won't start attacking it and to the enemy, the unit seems to have same unit colors and (it perhaps could even be selected), but no orders could be given.
3. An unit would have ability, to assume any enemy's shape by casting a spell to enemy unit.
How this could work? For example, if enemy clicks the disguised enemy, he could receive info that it is an enemy spy (and possibly force an attack to it). The skill could also have energy cost, when energy runs out, the disguise vanishes. Also, it could be detectable by detectors (and have "cloak" group), and if detected, its true form would be visible.
The possibilities are endless. What do you think?