EDIT: Uh... 
It also appears that you might need to change the name of Doppelganger (possibly by adding that 'accented' a).
http://elementscommunity.org/forum/index.php/topic,8700.0.html
Not necessary, because there are already other duplicate names in the Crucible and upwards. No one can claim a copyright on a name.
That said, umlauts are cool.
I think the ability has to be a passive since there are no previous cards with multiple actives. The name of the ability may be listed (Scarab) or not (Mummy).
Add
: Evolve to the notes.
Another description option for brevity: "Doppelganger's active ability becomes the most recently used active creature ability."
Like Burrow, I would assume SoR would no longer apply once the active ability is changed. For one-use abilities (Lycanthropy, Stone Form), I would assume it doesn't copy itself and get the ability repeatedly? If using Deja Vu, again I would assume the original is left blank, but then does the new copy gain Deja Vu?
This would combo extremely well with Steam Machine and another creature to cancel out the ability.
Oh, and this card is AWESOME. You got my vote.
