Reportedly Zack Snyder, Chris Terrio and David S. Goyer have already laid out a pretty solid storyline for the DCEU (took their time, one might say...) and it may just be coincidence but I thought that the selection of villains in the Suicide Squad film was an interesting way to build a spread out villain base for the DCEU. DC are being clever by not following Marvel's method of building their heroes one by one then mashing them together. They've decided to introduce us to their universe through a fresh, new hero (Superman) - and then chuck him together with already established (story wise) characters like Batman and Wonder Woman.
I can't help but wonder if the villains in Suicide Squad were selected for the purpose of universe building. The DC heroes will get solo films, but later on. This way we might get an insight into what those heroes adventures will be.
Batman is in the film himself, and apparently is the one who captured all members. As such there are many Batman antagonists in the film, The Joker and Harley Quinn are most prominent, but also Deadshot, Killer Croc and Jonny Frost. With rumors that Affleck will be in a trilogy of solo Batman films Batman 's villain pool is probably going to be quite large and with characters as popular with fans as Joker and Harley, Suicide Squad more than likely won't be their last outings.
The Enchantress is an interesting pick. She is an antagonist of Shazam in the comics and with a Shazam film planned for 2019 we might get some reference to him in the DCEU earlier than expected. Whether Enchantress will actually feature in the film or its plot is still obviously unknown. We do know that the main villain will be Black Adam, played by Dwayne "The Rock" Johnson however.
We have a traditional Flash villain in Captain Boomerang. The Flash is reportedly showing up in Justice League Part One before a solo film. Ezra Miller has been cast as The Flash but we don't know who he'll be facing off against in March, 2018 just yet. Maybe Captain Boomerang will show up, or maybe he'll just hint at the Flash in Suicide Squad until we get a look at the scarlet speedster ourselves in 2017.
A year or so ago David Goyer said DC would be releasing some "low budget" films to accompany larger films in the future. Truth be told, Suicide Squad was one of the names he mentioned. Who knows, Katana's presence might signal a Birds of Prey film somewhere in the distant future (filling the gaping hole in Marvel's universe of absolutely no female protagonists). There are endless possibilities and DC are doing a great job to set up their shared movie universe.