I personally like relays and wish the world relay championship was a bigger deal and put at a time where all the big names show up and are in peak conditions.
So while I agree that I wouldn't want this at the expense of other events I think it could have potential.
My thoughts would be to put it on the very last day like the marathon, this way all of the track and field program has concluded and you don't have anyone wearing themselves out in the relay before the main focus.
Then you form your relay pool from competitors in the 8-10k and pick two male and two females who are ready to go. Let them run in whatever order they want and get after it.
I'd imagine most of your traditional distance powerhouses have at least 2 male and 2 female athletes qualified in other events to form relay teams.
I would also think that USA could be competitive, who wouldn't have wanted to see Rupp get the baton neck and neck with Kam and Farah has to try to chase them down and make up the gap at London?