Even though Season 3 ended with Barry Allen (Grant Gustin) giving up everything to save Central City and be trapped in the Speed Force, no one truly believes that we'll get very far into The Flash Season 4 without seeing the hero return. Even the writers on the show have moved past talking about Barry in the Speed Force to what will happen just after he gets out. This looks to happen most likely in the very first episode of the season, like how last year's Flashpoint took place in the first episode and everything after was fallout.
The new season is supposed to be about a lighter tone, getting back to the fun of the earlier seasons — and the positive relationship between Barry and Iris (Candice Patton). Executive producer Todd Helbing talked about the eventual reunion with TVLine:
"There's a lot of love in the air this season. It's like somebody going off to war for six months and coming back. There's a lot that Iris experienced that Barry didn't when he was gone, and it's really [about] the fun and the emotional component of making up that time when they weren't together."
But there are other relationships on the show like Wally (Keiynan Lonsdale) and Jesse (Violett Beane), whose relationship will be going through an "evolution". Meanwhile, Cisco (Carlos Valdes) and Gypsy (Jessica Camacho) are "still together and will be for a while," even though Commacho is now a regular on NBC's Taken — and the appearance of her father, Breacher (Danny Trejo).
But the biggest change is that even though the Flash will have to take on the new big bad, The Thinker (Neil Sandilands), he's going to be dealing with a lot less emotional baggage, having left the angst-ridden Barry in the Speed Force, according to Helbing:
"When he comes out, he's sort of left that all behind."
Helbing also teased some changes for Killer Frost/Caitlin Snow:
"She has done some soul searching and figured out exactly how to deal with her demons."
That explains why we've been seeing a kind of hybrid Frost/Snow in the trailers and teasers.
The Flash Season 4 debuts October 10th on the CW.