If I could reinvent the DCEU from the ground up?
First up, no Zack Snyder. The man is just not a good fit for Superman, Batman, or the DCU in general. Second, there would've never been a Suicide Squad. They should've been focusing on building up to the Justice League, not introducing side projects like that that have nothing to do with Justice League (A mistake which they're apparently intent on repeating with their Gotham Sirens idea). So for me, my preference would've been as follows:
2013
SUPERMAN
The first part of a multi-film arc with Lex Luthor as the overall villain. Think of it as the Rise and Fall of Lex Luthor, with Luthor portrayed most closely to Lionel and Lex from Smallville and their emphasis on empire building, with Lex seeing himself as a modern Alexander the Great. Superman begins his career in Metropolis, slowly comes to the realization that Lex Luthor is not who he seems to be, and has to battle Lex Luthor's creation, Metallo. The movie ends with the defeat of Metallo, but Luthor comes away clean without anyone save Superman and Lois Lane even suspecting that he had anything to do with Metallo.
2014
BATMAN
We're introduced to a rebooted Batman. One who's been operating in Gotham for two or three years (as opposed to one who's at the end of his career, which is what we got with Batfleck). The first villain would be Clayface, and we'd play up the tragedy of Clayface's condition. The goal here would be two-fold. Firstly, having a story with plenty of pathos, as we'd have Batman facing off against a villain who has his sympathy, and which tragically ends with a confrontation which Batman doesn't want. And secondly, it puts Batman up against a super-powered opponent, thus firmly establishing him within a broader DCEU where superpowers are commonplace. This film could also be the place where Dick Grayson is introduced as Robin, whose introduction would revolve around the B Plot of Batman's war against the mobs in control of Gotham and Dick Grayson's family being caught in the crossfire.
2015
GREEN LANTERN
Hal Jordan is inducted into the Green Lantern Corps. Working alongside his partner, Sinestro, his first mission is to confront the resurgent Manhunters. This highlights the fact that the Guardians of Oa arent above making mistakes, setting things up when Sinestro is revealed to be a tyrant in a future film.
2016
THE FLASH
We're introduced to the world of the Flash, in which the Flash has been in operation for a year or two as well. The Rogues already exist as an organization with the Flash having regular run-ins with them, but the primary antagonist of this film is the Reverse-Flash. The costume would be truer to the comics than the TV show version, and much closer to the comics than the Snyder JL version.
SUPERMAN 2
The conflict between Superman and Lex Luthor continues as Superman is confronted with his first extra-terrestrial opponent, Brainiac. The film ends with Superman defeating Brainiac and exposing Luthor as a criminal, whos placed under indictment.
2017
WONDER WOMAN
Patty Jenkins did a perfect job. There's nothing I would change, save for the few bits of connective tissue to the existing DCEU. That's why I think a soft reboot of the DCEU which excludes everything that's come before save for Wonder Woman could work, since the movie is already effectively a standalone movie which doesn't require a universe to make sense.
BATMAN 2
Gothams criminal underworld, reeling from repeated assaults by Batman and Robin, take out a large contract on Batmans life, and several of the DCUs most dangerous assassins take up the contract. That includes Deadshot, David Cain, and Deathstroke the Terminator. Batman and Robin have to run the gauntlet through the most dangerous assassins in the world, culminating in facing off with Deathstroke, as they continue their war with the Gotham mobs. Along the way, they turn David Cains daughter, Cassandra, to their side, and find themselves with a new ally when they face off against Deathstroke. The interesting thing about this matchup is that we'd have Batman facing off against an opponent who is his intellectual and physical superior, and he only triumphs because he's not facing him alone, but with partners.
2018
JUSTICE LEAGUE
Lex Luthor is convicted, but he easily escapes custody. He then sets about a plan for taking revenge against Superman as well as take back everything he lost. To that end, he unveils his battle suit, puts together a team of super-villains, and puts his plan into motion, using recovered technology from the conflict with Brainiac (Or perhaps even allied with Brainiac) to give him an extra edge. Superman cant defeat them alone, and the plan is expansive enough that it draws the attention of multiple heroes, who then unite as the Justice League and take on Luthors Legion of Doom.
Maybe stick in an Aquaman movie in 2018, too, and then carry him over into the Justice League movie, similar to Black Panther and Infinity War. That would be my initial idea of what I wouldve preferred to have seen over what we got.