Every season of The Flash thus far has each ended on a terrific cliffhanger, which have kept us eagerly awaiting the show's return.
Season 3's might just have been the best yet, though. The finale, titled Finish Line, ended with the skies going red above Central City. The only cure was for Barry Allen to sacrifice himself and vanish into the Speed Force, effectively dying and disappearing into speedster heaven.
It made for a heck of a final scene to leave us on, and has left us pondering ways it could be resolved next season. There can't be anyone out there that truly believes this is the last we have seen of Grant Gustin on the show - The CW are hardly going to let the star of their most popular superhero TV series go, are they? So the mystery surrounding the finale and Season 4 is not whether or not Barry Allen will return, but just how that's going to happen.
In a show with as rich a mythology as The Flash, there are myriad ways for the Scarlet Speedster to return from the Speed Force, each of which would affect him differently as a character and change the shape of next season.