The DC Extended Universe has gone through several attempts to make a movie about the Flash. Their first attempt to add him to a shared franchise was a script written in 2013, and apparently, it would have included Green Lantern at the end.
“We did a Flash screenplay and Hal Jordan showed up at the end of Flash,” said Marc Guggenheim, who co-wrote the script with Greg Berlanti.
The two more recently became the co-creators of Arrow and The Flash TV series, respectively. (Which do share continuity in the “Arrowverse,” though no Green Lantern has ever shown up). Of greater note here, however, is that the pair also co-wrote the screenplay for the 2011 Green Lantern movie, implying that the two films were perhaps meant to be connected.
It is known that DC Comics and Warner Bros. originally planned for the Green Lantern film to be the foundation of their shared universe, but those plans got nixed when it turned out to be a critical and financial dud. 2013 instead saw them re-start the franchise idea with the Superman-focused Man of Steel.
The dour tone of that entry was divisive, but it was a big moneymaker. The franchise has continued, but unfortunately, every entry except Wonder Woman has received poor critical and/or audience feedback.
Putting Barry Allen and Hal Jordan together in one movie makes some sense, as the two are often depicted as friends in the comics. It remains to be seen if they will have a similar dynamic if and when they show up in the DCEU.
Since rejecting Guggenheim and Berlanti’s idea, Warner Bros. have gone through several other scripts and directors for a Flash movie, finally seeming to settle on an adaptation of the 2011 Flashpoint event from the comics, using that title. There are also plans for a Green Lantern Corps film, which will star Hal Jordan aside the other human Green Lanterns. No release date or directors are set for either of these projects, however.
Perhaps a pity—even if it would have made the less-than-stellar Green Lantern film canon, fans could have probably expected a good Flash movie from the pair, as the TV series, now in its fourth season, is quite popular.