Hey, I would like to first say thanks for reading my first post! I have been thinking about this for quite some time, the Flash (Wally West in particular) is one of my favorite characters. So, without further adieu, this how I would handle the Flash in the DCCU.
The Justice League Part One:
While we will be getting our first official Flash movie at some point in 2018, the character (or characters) will likely be introduced before hand! Probably in the Justice League Part One.
As Brainiac launches his invasion of Earth, Barry Allen leaves Kid Flash and Central City to join the newly formed Justice League. The Justice League, in it's first outing, is pushed to it's outer limits; each member struggling to stand against the might of Brainiac and his invading machines. At some point in the battle Barry Allen manages to reach Brainiac's Ship and runs at speeds he has never before reached. The kinetic energy that Barry is producing is enough to bring the ship low and give the League the chance to strike back; though, it comes at a cost, Barry Allen begins to fade away as he is forced to join the Speed-Force (á la Final Crisis). Wally West and the rest of the world simply watch as Barry Allen seemingly ceases to be.
The film would end on a rather somber note. Brainiac was successfully decomissioned, but the cost weighs on the League. The Mayor of Central City declares that a Flash Musuem will be erected to honor of the fallen hero, and Wally West is left without an uncle and mentor.
The Flash(2018):
The film opens with Wally roaming about the Flash Museum aimlessly with Linda Park, trying to decide what comes next; he had never really been the one calling the shots. Barry Allen is obviously missed by the public, his death weighing heavily on all of them.
Though, not everyone is upset over the Flash's death. The Rogues-Captain Cold, Mirror Master, and Heatwave-celebrate the death of the Flash within the walls of Iron Heights. A prison break would be staged by the one-time memeber of Task-Force X, Captain Boomerang, and the Rogues plan a strike on the Flash Museum.
Unbeknownst to any of them, William Dawson (Grodd) managed to escape during the chaos aswell.
Learning of the Rogues intent, Wally knows that he must stop them from damaging his uncle's memory. Pushing himself harder than he had ever before, he manages to reach speeds that rival those reached by Barry Allen (before he was vaporized). Wally then adopts the Flash costume and confronts/defeats the Rogues as they launch their attack. After all of this, Wally officially takes up the mantle of the Flash and joins the Justice League.
The Flash(20??):
Following the events of the Justice League Part 2, Wally and Linda are now engaged. Wally is as confident as he will ever be, having made several friends in the local police departments, most notably Hunter Zolomon, an ex-FBI profiler who has helped the Flash catch the likes of the Trickster.
While it seems that peace has returned to the Twin-Cities, it does not last. Gorilla Grodd, having managed to fully shed his human-persona (William Dawson) leads an invasion force to Keystone and Central City that is meant to finally kill the Flash and any who would wear the his golden boots.
At some point in the battle, Grodd manages to get the better of Flash. Hunter would come to try and save Wally, but Grodd would get the better of him; Hunter calls out for Wally's aid, but Wally cannot come and Hunter is nearly killed by Grodd. Though the Flash does eventually manage to get both himself and Hunter away from Grodd, Hunter is left crippled. Eventually, the Gorilla invasion would break and Wally is able to get the better of Grodd and drive him out of the city. Though the public praises the Flash for saving their city, there are many who feel let down-mainly Zolomon. The Flash could have prevented all the destruction if he was just willing to push himself like Barry did.
At any rate, the film closes with the marriage between Wally and Linda, peace returning to the city.
The Flash(20?!?!?!):
Opening with Hunter Zolomon wheeling his way into the Flash Musuem, we get a glimpse of some of the new additions: A statue of Wally West as the Flash, the Kid Flash's Costume and Gorilla Grodd's Crown. After that little showing, we would get a glimpse of Professor Zoom's costume.
Eventually, Zolomon would reach a now rusted over Cosmic Treadmill. As he attempts to activate it, he triggers a chain reaction that results in a massive explosion that levels the entire Musuem. All we are left with is a massive crater and no sign of Hunter.
The Flash would arrive at the scene, in utter shock. There was no way of telling who did this or how it even happened. After searching through the rubble, he would find what seems to be a hospital armband and the melted down remnants of a wheel chair. Wally races from hospital to hospital, trying to find out which one has a missing patient. He finally learns that it is Hunter who is missing. As he realizes this, the hospital he is in is shaken by one sonic boom, then another, and another. The Flash tries to move as many people out of the building as he can before it collapses.
As Wally searches for whoever was responsible for the attack on the hospital, he is greated by "Zoom." The new Reverse-Flash taunts Wally in ever shifting tones and procedes to quickly beat the Flash. Eventually, Zoom finds his way to Linda and nearly kills her.
Seeing what he is capable of and what he is willing to do, the Flash throttles the unexpecting Zoom and forces him some distance away from the city. Zoom allowed this to happen though, as he once again began to beat Wally, saying that he needs to be better. It is then that he reveals himself to be Hunter Zolomon and warns Wally that, if he cannot catch and stop him like Barry stopped Thawne, then he will make sure he kills Linda. Zoom bolts, becoming a blur even to the Flash, but the Flash continues to push himself. Like Barry before him, Wally would begin to rapidly age and slip into the speed force; however, keeping Linda in his thoughts allows him to remain grounded. Eventually he catches Zoom and manages to stop him from finishing off his wife.
The film ends with all the rainbows, lightning and babies you could possibly want. I can't write anymore!
Now, these past few paragraphs have been only "brief" outlines of how I would handle the Flash in the DCCU. Obviously there are going to be plot-devices that need ALOT of explaining. For example: The Cosmic Treadmill, Professor Zoom and Barry Allen, Gorilla City, etc... I hope you all liked it. Please leave constructive criticism.