[Warning: This story contains spoilers for Warner Bros. and DC Entertainment's Justice League.]
The final post-credit sequence for Justice League teases something at once both obvious and exciting for long-term comic book fans.
Lex Luthor (Jesse Eisenberg) has escaped from prison and meets with Deathstroke (Joe Manganiello), suggesting that now that the good guys have a team, maybe the bad guys should think about doing the same thing, too.
This is, of course, hardly the most original idea in the world — Lex, you're supposed to be a genius! — so it should come as no surprise to discover that the Injustice Society of the World debuted in All Star Comics No. 37 in 1947, just a few years after the first appearance of the original comic book super-team, the Justice Society of America. This launched a trend for Justice League villains to add a prefix and stay on topic when coming together for the greater good… well, okay, greater bad.
In addition to the Injustice Society — which has gone through multiple iterations — there have also been two different Injustice Gangs and two different Injustice Leagues, as well as the off-brand Secret Society of Super-Villains (again, two different attempts at that) and the Legion of Doom, the subtly named opposite number for the animated Super Friends in the 1970s and '80s. In each case, while the names may have changed, the purpose has been the same: a supergroup of villainy, with the arch-nemeses of the big-name heroes putting aside their differences to cause trouble and try to get rich in the process.
Although the lineup of such groups shifts with each incarnation, there are enough commonalities and constants to know roughly what to expect from a cinematic Injustice [Collective Title Here]. Firstly, of course, Lex Luthor will be at the center of things; even putting aside the fact that he's one of the few recurring villains in the DC Cinematic Universe at this moment — outside of Suicide Squad's incarcerated ne'er-do-wells — his ego is simply too great to allow him to be anywhere else. Of course, that ego is likely to be not only his undoing, but also the group's undoing as a whole, but you have to take the rough with the smooth, apparently.
Similarly, if the Joker is involved — a possibility, considering Jared Leto's version is running around following Suicide Squad — then things will get out of control sooner rather than later. (The 1990s JLA storyline "Rock of Ages," which featured the second Injustice Gang, saw the Joker take control of a reality-altering object, thereby threatening the existence of… well, everything. That's kind of how these things tend to happen.) This ties in with the third and most concrete of Injustice collective tropes: that any group of villains will ultimately prove to be their own undoing, although such an event will nonetheless still allow for a climactic showdown with the good guys.
The Legion of Doom is one of the few villainous collectives to last beyond one single storyline without a major falling-out or reorganization, mostly due to the demands of the Saturday morning animation model of storytelling; every other grouping ends up being temporary, with an inevitably explosive conclusion, which is one reason why an Injustice grouping makes sense for a Justice League sequel.
A second reason to journey down this route is, essentially, bragging rights: Marvel may have done the first supergroup of big-name superheroes with 2012's Avengers, but depending on plans for Avengers 4 and the speed with which Justice League 2 gets underway, Warner Bros./DC could beat them to the punch with the big-name supervillain team-up. Petty? Perhaps, but it'd grab the audience's attention, which matters in an increasingly crowded marketplace.
Moreover, an Injustice League movie is probably what Lex Luthor would want, if we're being honest, which is almost reason enough in and of itself. If Justice League was all about being "All In," as the tagline put it, Justice League 2 needs to be about being "In Cahoots." After all, it's good to be bad.