Metamorphmagus-twist in DH

[Inge]

No: HPFGUIDX 165620

Wondering if we will have a twist in DH similar to the twists in PS, 
CoS, PoA and GoF.

Im referring to 
Quirrel = Voldemort (well, sort of), 
Diaryboy = Voldemort, (plus introduction to polyjuice to play out in 
GoF)
Scabbers = Pettigrew, 
Moody = Barty Crouch

All of these seemed to be someone other than who they really were.
We didnt have that twist of mix-up in OotP but we were introduced to 
the Metamorphmagus, Tonks.

Seems strange to me that Tonks' abilities didn't make much 
difference to the plot of OotP or HBP, and my guess is, she was 
introduced for a reason much like polyjuice in CoS; and that 
Metamorpmagus will play it's real role in DH much like polyjuice 
came to play that larger role in GoF.

So, what about Metamorphmagus in DH? 
Will it turn out that Tonks is not only Tonks but also someone else? 
Someone already known to the readers. If so, who?
Logically this other person, whom Tonks changes into, must be 
someone who is never seen in the same scenes as Tonks.

And if the Metamorphmagus-twist in DH isn't about Tonks, then who 
else might turn out to be a Metamorphmagus, who through the entire 
series has also acted as someone else, using these abilities?
Again, logically we'll have to look for two persons who are never 
seen together. 

Inge