Everyone should be able to double or triple - leave it to the IAAF to throw away the chance of positive media headlines for the sport about such. But the problem is the WC format, compressed as it is to one week, it should be streched over many weekends.
Weekend 1: The 100 meters.
Weekend 2. The 200 meters, with all the finalist from 100 as wildcards.
Weekend 3. The 400 meters, with all the finalists from the 200 as wildcards.
Weekend 4. The 800 meters, with all the finalists from the 400 meters as wildcards.
Etc, etc, etc
The IAAF should stretch the World Champions title battles out over the whole second half of the summer to maximize media attention - AND that way it would also be possible to keep the Diamond League running in the background AT THE SAME TIME!