You’re waiting on a brief. Everything is in flux. Nobody knows nothing and panic runs in the streets. You know what you do? Start. Do not, I repeat, do not, get caught up in the swell of anguish and moaning. The reason that sort of thing gets so much momentum is because it’s easier than shutting the door and starting. Look, you know enough about the client or the brand. You have a basic sense of the need. Dig around on your own, write your own mental brief, and start solving in parallel to the official brief.
The chances are pretty good you’ll be in the ballpark in the end. You see, the problem everyone really has is time. Moving ahead now buys you more of that when the real brief finally shows up. You’re not wasting time starting early. You’re gaining it.
Or sit around and bitch with the others. Your call.