2) Wally West
It may be a bit early in Barry Allen’s superhero career to take on a sidekick (or die and get replaced), but considering that Wally West is one of the most important characters in Barry’s life – and took on the Flash mantle for nearly two decades – you simply cannot make a Flash series without introducing him at some point.
Wally West is the nephew of Iris West, and takes on the moniker Kid Flash after being struck by the same lightning/chemical accident that gave Barry his powers. When Barry sacrifices himself to save the Earth during the Crisis on Infinite Earths storyline, Wally takes up his fallen mentor’s costume and superhero identity.
We know that the creators of the show want to introduce Wally into the show, so at this point it’s only a matter of time. As I said, it’s a bit early in Flash’s career to be taking on a protege, but Arrow introduced Oliver’s sidekick, Roy Harper, early on in the series and then transitioned him into being a costumed crimefighter at the end of the second season. Wally could easily show up as a side or supporting character and then make the transition to Kid Flash when the time is right.