Thanks for dropping by Alicia.
IMO, Google's biggest fight would be against everyone who wants to guard privacy. Well, we know that privacy is gone the instant we connect to a network.
The way Google is becoming part of everything that constitutes our life - phone, car, friends and every other thing, that makes people more wary of the way how our details could be used by Google.
Guess, that would be it's biggest challenge, keeping private data 'private'