3) Another Green Lantern
On the other hand, it doesn’t have to be Hal Jordan who appears in Justice League. As the title of the movie suggests, Green Lantern Corps will star many different Lanterns. There are definitely a heap to pick from, too. John Stewart, Guy Gardner, Kyle Rayner, Jessica Cruz, Simon Baz…the list goes on.
Alternatively, it could be one of the many alien Lanterns. If Corps wants to deal with the first human Green Lanterns that would make a lot of sense. Justice League could introduce the organization before we properly get to know them in a later movie, then.
Again, we have it on pretty good authority that an alien Lantern will appear in some form. We know it isn’t Kilowog, but that still leaves the door open for other notable members. For instance, Tomar-Re or even Abin Sur, the Lantern who gives his Power Ring to Hal Jordan.