I have predicted to @Admiral James Kirk elsewhere that I believe that Wally and Barry have been stuck in the Speed Force all of this time, and that Bart (who is actually Bart Allen, Impulse) has contained fragmented memories of these people, which is why he used their names as aliases. Miller will reveal that he's actually from the future, he's already implied that he arrived with a murky memory in an earlier flashback. Miller needs to write a Flash book now that he's proven he has a handle and care for the Flash mythology.