Shape-shifting, the Underestimated, Game Breaker Super Power.

As I have just recently completed the newest Mistborn Novel, Shadows of Self, I have had my mind drawn back to the Kandra, somewhat Humanoid beings that can take on the shape and form of any being that they digest the bones of.  The extra twist with these abilities is that due to a combination of immortality and extreme cunning, Kandra are masters of wearing the mask.  They are so skilled at their craft that they are able to masquerade as the being they are impersonating to the point where they can pass sometimes for many years without being caught. (They rarely ever get noticed.)

Shape Shifting tends to be a stock superpower that never reaches the game breaker levels it is capable of.  Partially this is author lack of foresight in this department.  Partially it is also due to the writer's wishes to develop other concepts.

Still, when in the off chance, a writer uses the concept correctly, the ability to up the ante and intensity of a story increases as the abilities of shape-shifting are fully explored.  

The paranoia fuel that is capable with shape-shifters is without few to compare.  The only examples I can readily come up are planting bugs on a person's body, watching through their shadows to be anywhere, or linking their brain to a greater hive mind (and lesser forms of mind control)

When characters are forced to use code words to identify each other and will not share secrets with their closest confidants, that is when the idea is being used correctly.  When parents see their children as potential killers and vice versa, this idea is being utilized to its greatest potential.

There are other secondary abilities that can be tacked onto Shape-shitifng, or create all kinds of new (and sometimes kinky and squicky) situations.  Some examples of this are a shape-shifting seducer that in its true form is actually an Eldritch Abomination.  One of my favorite examples of this concept is the Yaclol which in its true form looks like this:

It other forms tend to be more akin to the mythological Succubus (in fact, in the setting where these things exist, they once were Succubae) 

But there are other non squicky concepts one can use with shape-shifting.  One which I am using heavily in the Search for Eden books.  When brought down to more of a science, some forms of this ability are a complete rewrite of cells.  If this is so, its not unheard of for healthy amounts of regeneration to be a side effect of such powers.  Regeneration of this magnitude makes shape-shifters incredibly hard to kill.

If you enjoy combinations of paranoia fuel and squickly situations, I highly recommend reading Mistborn by Brandon Sanderson. 

I also recommend of course, my own writings which for sure also have these elements.