An Instagram user posted the following image revealing Wentworth Miller as Captain Cold; Brandon Routh as the Atom; Caity Lotz (as maybe a time-resurrected Canary?); Ciara Renee as Hawkgirl; Victor Garber as one-half of Firestorm, Victor Stein; Stephen Amell as Arrow and Grant Gustin as The Flash.
The Instagram user also claims Rip Hunter is present (played by Arthur Darvill), but he's not seen in the image.
Now for some speculation:
Previous images from the episode revealed Ollie in a new League of Shadows Arrow costume, and now we see Ollie back in costume as well as Caity Lotz is back. We know The Flash can time travel, and there is also the time traveller Rip Hunter. It's probably not too hard to guess that something might happen regarding time travel to bring the team together. It's possible Caity Lotz may be plucked from a point in her past, but not necessarily messing with her death, as she may go back to the same point in time, "eventually." This way would then not cause some sort of time paradox and wouldn't undermine her Season 3 death, but it would enable the use of the fan-favorite character and actress (another possibility is she could be from a parallel universe).