A single piece of large graffiti listing full instructions? I can’t even begin to count the number of potential paradoxes this would cause. The Bad Wolf message was delivered through suggestion. It’s short and simple and ambiguous (see previous posts) and it only means something to its creator. Whereas even a short message like “Rose: Save the Doctor” would make no sense to anyone else (why am I writing this/calling my helicopter/power station this?) and if the doctor came across it (as he did the other Bad Wolves), whoops – another paradox! Also, as someone over at Outpost Gallifrey pointed out, “Rose: Save the Doctor” would be a rubbish name for a TV station.