Thanks to comicbook.com, this video titled "Justice League Retro Trailer" has gone viral. Features Michael Keaton's Batman, Christopher Reeve's Superman, Lynda Carter's Wonder Woman, and John Wesley Shipp's The Flash against Gene Hackman's Lex Luthor and Jack Nicholson's Joker.
Shipp's Flash we've seen plenty of times in the Arrowverse, through his 1990s adventures on Earth-90 as well as the crossovers Elseworlds and Crisis on Infinite Earths, the latter of which is Flash-90's end. Keaton and Nicholson are supposedly the Bat and Joker of Earth-89. Christopher Reeve and Lynda Carter only made cameos as newer characters in the Smallville and Supergirl series. Speaking of, there's even a Helen Slater cameo at the end.
Check it out...
Probaby considered off topic, but I would think worth a check out given its connection to the overall multiverse. What could have been, if you will.