The Flash Season 3 ended with Barry Allen leaving his friends and family behind in Central City, and voluntarily walking into the Speed Force. Details about how the Speedster will return in The Flash Season 4 are scarce, but we know that actor Grant Gustin is onboard for the CW's show upcoming season.
A report in TV Line has confirmed that villain in season 4 will be Clifford DeVoe aka The Thinker. Unlike other seasons, this season's villain will not be a speedster, as showrunner Andrew Kreisberg revealed in March at PaleyFest, "Next season we're not going to have a speedster [as the Big Bad]."
Here are a few fan theories on what's next for Barry Allen in The Flash Season 4:
1. Wally West becomes the new Flash:
A fan theory suggests that Barry will be inside Speed Force for long, as Wally West becomes the new Flash in Central City as it happened in the Comics.
During the Crisis On Infinite Earths event, Barry sacrificed himself to stop the Anti-Monitor's weapon, and his death was followed by Wally West taking up the Flash's mantle. A Reddit user notes that Wally should do the intro wherein the Kid Flash says, "My name is Wally West and I'm the fastest man alive. A year ago, a future evil version of my foster brother Barry Allen gave me powers to match the ones I had in an alternate timeline. Barry defeated his future evil self, but now he's trapped in the speed force. One day I'll find a way to free him so he can marry our sister. I am The Flash."
2. Barry is the new villain: The Barry, who returns from Speed Force prison, may not be a hero. According to a fan theory posted in SceenRant, the Scarlet Speedster, much like Hunter Zolomon, will have to pay for his past mistakes against time. For Zolomon, this meant becoming Black Flash, who is the grim reaper for speedsters, and it could be the same fate for Gustin's character for creating Flashpoint.
The website has speculated that with Black Flash' death in Season 3 finale, the Speed Force could enlist Barry to be its new Grim Reaper. So Allen returning from the Speed Force as its new and unwilling Black Flash and the entire Star Labs team rallying to help him could be a plot possibility for season 4.
3. Barry returns from Speed Force with new powers: The CW writers could take cues from Terminal Velocity Comics, where speedster Wally West's mastered the Speed Force and used it to unlock brand new abilities and control over speed, vibration, and in some cases, matter and time itself.
In season 4 of The Flash show, Barry could be something of a new man, having gained a far greater understanding of the powers he wields, while simultaneously gaining a clearer for bond with the Speed Force itself.