An opposition won't be necessary. The application will 99% get rejected, since the term "bitcoin" will be deemed descriptive or even generic. On the small chance it slips by the examiners, then an opposition would be in order.
Not sure I agree with this. If you're saying that the word is too common to trademark. There's always the first person to file the trademark for a term. And they're the ones who get it. And Bitcoin wasn't even a word 3 years ago. Let alone trusting the evaluators to be familiar with it even today ... It will be rejected if someone else has trademarked it but not sure it would for any other reason?
This isn't how trademark law works. All you need to prove in order to nullify a trademark application is to show that SOMEONE, SOMEWHERE used the term before that person did. You could also argue that the term is generic, or a myriad of other defenses. You don't need to have a "registered" trademark in order to have standing. Anyone who has printed the word "bitcoin" on a t-shirt has standing for prior use. And anyone can oppose on grounds that the term is generic.
IANAL but....
Once someone has a trademark they can be a pain in the ass. A patent works more the way you say then a trademark. A trademark should not be granted for this, it is invalid, but if it is it is hard to defend against.