I've so far been grateful that J'Onn J'Onzz has been spared involvement in Warner Brothers' lousy shared universe of motion pictures, but the hope is that Wonder Woman will mark a course correction. This is especially true now that the new animated opening from the DC films has been released, and pretty clearly features the Manhunter from Mars in a relatively prominent position.
If you look at the final Cinemascope view of the featured characters, there are roughly five groupings, with the center section dominated by the movie Justice League. Specifically, that's The Flash, Wonder Woman, Superman, Batman, and Aquaman, all of whom also got solo animation spotlights earlier in the clip. J'Onn J'Onzz's replacement as a League founder, Cyborg, also got a solo spot, but is inexplicably consigned to the fore of the second grouping at left of the JLA (subtext?) Green Lantern is also way in the background of the core group, flying upward, along with that classic icon... Atrocitus?
Ewww-kay.
In the group at right of center are a bunch of silhouettes that appear to resemble other Lantern Corpsmen, with (I think) Sinestro the most prominent but also (probably) involving Jessica Cruz and Kilowog. Less ambiguous are Nightwing (back to his blue chest symbol,) Harley Quin, and our own Alien Atlas!
...and the Green Lantern central battery. The DCEU is very invested in GLs.
How big of a deal is this? Difficult to say. J'Onn is clearer and more visible than many of the characters here, and the more obvious inclusions all have representation in films, television, and/or animation at present. His appearances on Supergirl (also included) would seem enough to qualify him, given the presences of DC's Legends of Tomorrow's Firestorm, Hawkgirl, and Hawkman (as well as Green Arrow and DC Super Hero Girls' Bumblebee.) Still, confirmed film stars like Mera, Batgirl, Shazam and Black Adam are of similar standing in the intentionally vague group shot (as is TV's Black Lightning, admittedly) so place your bets and we'll see how it pays out...