One noticeable difference between the Marvel Cinematic Universe and the DC Extended Universe is that the former has expanded with a large presence on television and Netflix. The latter, perhaps controversially, maintains that the DC movies and TV shows will be separate, even though there's a very successful version of The Flash currently running on the CW. It's understandable why they would do this, since it allows the various creative teams more leeway when crafting their projects.
Still, fans remain hopeful that one day DC Entertainment will cross the streams, combining their small screen and big screen productions. DC Creative Chief Geoff Johns teased the die-hards with the idea that everything forms a multiverse, or a collection of parallel universes where different events happen to the same people and characters. It's a concept DC Comics is known for, and perhaps one day it's something they'll fully embrace.
On his Fatman on Batman podcast (hat tip CBR), Kevin Smith discussed the recently aired DC Films Presents: Dawn of the Justice League special that gave fans exclusive looks at Warner Bros.' upcoming film slate (which Smith co-hosted with Johns). The filmmaker talked about a segment that was cut from the final product, which involved him asking Johns about the possibility of a multiverse being depicted on-screen:
"We did a piece where we talked about -- I don’t know how much I’m supposed to say because they cut it out. But the idea [came up] of, 'Hey man, DC is known for nothing if not a multiverse,' so at which point I was like [to Johns], 'Could they f***ing crossover?' And he was like, 'Ahh.' So doors are being left open and stuff like that."
This is far from confirmation that such a crossover is in the works, but Smith's recollection of the conversation implies that it's at least on the table. Conspiracy theorists will get a kick out of the fact this particular bit was left on the cutting room floor, perhaps a decision made purposely to preserve the surprise of a multiverse reveal. On the other hand, the TV event was geared more towards casual audiences, providing brief introductions to heroes like Aquaman and Cyborg. It's possible that talk of a multiverse would confuse those viewers, particularly about what they should expect when The Flash solo film hits theaters in March 2018. WB/DC are making a concentrated effort to emphasize the movie and TV show are not connected, and a multiverse plants a different seed in audiences' minds.
There's always a chance that one day Ezra Miller and Grant Gustin will portray Barry Allen together, but it's not something that's going to happen anytime soon. Johns has previously said that any links between the films and TV series would have to be logical and organically serve a narrative. For now, his biggest concern is to "let the stuff breathe and be its own thing," which is the smart way to go. DC is only beginning to piece together their film universe, so they want to see what exactly they have and how fans react to certain characters before diving into anything too out there like parallel worlds. It's best to start off (relatively) small on the outset.
Not that this should be a big deal. Two variations of the same character existing in different mediums has happened before. The TV show Smallville (starring Tom Welling as Clark Kent) ran from 2001 - 2011, a time span that saw the theatrical release of Superman Returns (with Brandon Routh in the lead role) in 2006. It's true that things have changed since then, and audiences expect fully blown "shared universes" in their pop culture entertainment. But not everyone watches the Flash on TV, so going in a different direction for the film should help the big screen version reach a wider viewership. And it also allows WB/DC to possibly get creative down the road with a multiverse, which would make it all worth it.
Batman V Superman: Dawn of Justice opens on March 25, 2016, which is followed by Suicide Squad on August 5, 2016; Wonder Woman on June 23, 2017; Justice League Part One on November 17, 2017; The Flash on March 23, 2018; Aquaman on July 27, 2018; Shazam on April 5, 2019; Justice League Part Two on June 14, 2019; Cyborg on April 3, 2020; and then Green Lantern Corps. on June 19, 2020.