The Justice League after credits scene took fans by surprise, and there's one man to credit for it.
That would be director Zack Snyder. Snyder left the project due to a personal tragedy, giving the reigns to Joss Whedon, but a new photo from Snyder's Vero account reveals he was still in the director's chair for the scene that introduced Deathstroke to DC's film universe.
The photo shows Snyder talking to Joe Manganiello, who is rocking his Deathstroke costume. The scene shows Deathstroke arriving at a yacht (seen in the photo) to meet a mysterious second party. That turns out to be Lex Luthor, who has a proposition for Deathstroke.
Deathstroke takes his helmet off, revealing Magnaiello's crazily comic accurate portrayal. At that point Lex brings up the Justice League, saying "shouldn't we have a League of our own?"
It seems this sequence was always part of the plan then, suggesting that a Justice League 2 was planned to feature the League of Doom. It was left in the movie even after Snyder's departure, so odds are it will continue to be a focal point of the sequel.
That said, the status of a sequel is a bit in limbo at the moment.
Justice League 2 is a bit mysterious, though thanks to J.K. Simmons' recent comments fans know a script for the project is being worked on at the moment. Early rumors suggested that the first Justice League was almost a prologue to a sequel, where the League would face down a long teased Darkseid.
That doesn't seem to be the case now, and frankly, a League of Doom conflict might be a much better route to take in the sequel. You could use the time to build up a bit more about Apokolips and Darkseid's other minions like Desaad and Granny Goodness (if they are going to use her), and would easily allow for the addition of Green Lantern.
Regardless of which direction they go in for the sequel, one can only hope it includes plenty of Deathstroke.
Justice League is currently in theaters.