1) Jay Garrick
Barry Allen may embrace and symbolize “The Impossible” in his series, but he was not the first man to don the Flash moniker. That role belongs to Jay Garrick, who first appeared in the comics in 1940.
Garrick is a college student who fell asleep in his laboratory and inhaled hard water vapors and developed the power of superspeed as a result. He becomes a college football star for a bit and then takes up heroics by donning a red shirt with a lightning bolt, and a stylized helmet with wings, reminiscent of the Roman god Mercury.
Garrick is a founding member of the Justice Society of America, an early iteration of the Justice League, and teams up with other iconic Golden Age heroes like Green Lantern, Wildcat, Hourman, and Hawkman.
Because of the way Barry’s powers have been established as the first superpowers to appear in the Flash/Arrow universe, introducing someone like Garrick would be a bit tricky. However, we saw the JSA in Smallville and their introduction was handled with believable care, so it’s certainly possible.
In any case, seeing an iconic character like Garrick would help establish a greater DC TV universe and offer up a fun opportunity for fans to see these two iconic characters on screen together.