We don't know much regarding Season 4 of The Flash, but we do know Barry Allen is on his way into the Speed Force. As for what happens after, we've compiled a few guesses as to how The Flash handles Barry during his time inside. Take a look at our best guesses for where this new experience takes Barry in Season 4:
Barry Stays Inside The Speed Force For A Long Time
Barry enters the Speed Force, and while he seems to be gone for only a day and a half, the whole thing unravels like a Clive Barker novel when Barry realizes he's been gone for YEARS. The new baddie is well established in Central City, Team Flash is fighting a losing battle but still around, and Barry Allen has a hill to climb before he can even begin to dismantle this chaos.
It would be fresh for The Flash to begin with the villain already out in the open for Season 4, but you'd have to wonder exactly what Barry could do that three decently trained speedsters (assuming Jay and Jesse are around to help Wally) couldn't. Of course, the show wouldn't be named after him if he wasn't the best, so it's possible this is how Season 4 rockets off.
Barry Gets Rescued By Team Flash But At A Cost
Barry enters the Speed Force, and, conveniently after gaining some world-changing perspective via his time inside, Team Flash rescues Barry. They managed to snatch Wally out of there pretty quick with Cisco's help, so can't they just do it again? The only issue is that one of our speedsters is going to have to go in for a bad time, and Wally and Jay have already had enough.
Perhaps Jesse Quick steps up to the plate, so Earth 2 Harrison Wells has reason to stay, and The Flash writers don't have to juggle Wally's relationship with Jesse in addition to Barry and Iris'. Obviously a raw deal for Jesse, but the world isn't exactly bubbling over with speedsters to keep the Speed Force from going nuclear.
Barry Becomes Black Flash
Barry enters the Speed Force, and much like Hunter Zolomon, will have to pay for his past transgressions against time. For Zolomon, this meant becoming Black Flash, who is the grim reaper for speedsters. Many theories have speculated that with Zolomon's death (which came via Killer Frost's ice blast when the time portal opened), the Speed Force will enlist Barry to be its new triggerman.
The only issue is, with Reverse Flash, Zoom, and Savitar dead and Barry in the speed force, who is left to be punished? As of Season 3 that would leave Barry with the task of killing either Jay Garrick, Wally West, or Jesse Quick. My money would be on Jay, considering he's not all that important to the central story, plus Barry killing a guy that looks like his dad is right up The Flash writer's alley.
Barry Didn't Actually Go In The Speed Force (Time Remnant)
Barry enters the Speed Force... OR DID HE?! The old speedster switcheroo is in full effect as Barry creates a time remnant to enter the speed force in his place. Why any time remnant would sign onto this is beyond me, as their existence from start to finish would be absolute misery. For Barry, he would have gone from thwarting Savitar only to recreate him all over again by damning yet another remnant to a meaningless existence.
This is, admittedly, the least likely outcome of The Flash as it would contradict the entirety of Season 3 and make it all for naught. Then again, Barry was dumb enough to recreate Reverse Flash with Flashpoint, so is it really a stretch to think he won't recreate Savitar again through a mistake?
Barry Becomes "One" With The Speed Force
Barry enters the Speed Force and then becomes it. It sounds crazy, but in the comics, Barry Allen is the end all be all in regards to the mysterious plane. Without Barry Allen, the Speed Force does not exist, and through Barry Allen, the Speed Force grows in influence. The Flash series has done little to support this comic book canon, which states the Speed Force was created when Allen was hit with the lightning bolt in his lab.
Perhaps in embracing the Speed Force Barry learns to stabilize it and co-exist without the sacrifice of a speedster. While a cop out, Barry Allen's dominance and control over the speed force would be a convenient and handy way to close the door on the issues so fans aren't always concerned with what speedster is currently inside and serving time.