Reading through Golden Age comics is a favorite pursuit of mine lately. During Grant Morrison's run on Batman, I scoured over a hundred old stories looking for clues and Easter eggs, and found, at the very least, a lot of inspiration for elements in his RIP tale.
Lately, I've been reading up on the early adventures of the superheroes who eventually became the JSA, and found an interesting parallel between the current Flash Rebirth story and an extremely old story.
In Flash Comics #2 (the second Flash adventure, starring Jay Garrick), the Flash intimidates an opponent by flinging a metal lightning bolt into the wall. While those early Flash stories had a light-hearted feel to them, with especially cartoonish faces for many of the secondary characters, there was a lot of death, too. Being the Flash's enemy was not an especially good idea. Some of his enemies die, and he resorts to physical intimidation frequently.
The metal lightning bolt reminded me of the weapon used by the mystery villain in Flash Rebirth #1. It is mounted on a shaft, like a spear, but otherwise is a pretty similar artifact to the one Jay Garrick wielded back in 1940.
A clue? Probably not. More likely a case of "great minds think alike." It seems incredibly unlikely for Jay Garrick to be revealed as a villain. But if it is a clue, here are the two points that connect the line.