What if it had been a Superman instead of Supergirl?
the studio obviously thought Keaton would be the big name of the movie (and rightly as Batman 89 was a phenomenon), but I guess beyond the hard fans the general audience just didn't care or they figured it was more about The Flash (obviously) than Keaton Batman. But since the studio figured Keaton Batman would be the big name draw it no matter so much about an unknown Supergirl to round off the 3 main characters
, but what if thered been a SuperMAN there instead .. obviously not Cavill as the threat of the story relies on the fact they haven't got his ultra powerful version there to take on Zod, so a much less powerful Superman, drained by long term exposure to kryptonite , not trained up over 30years, and had been held a prisoner (like SG was in the movie).. I'm thinking the Flash and Batman get to Siberia open up the kryptonite prison cell and find... Nicolas Cage Superman! (which would make all kinds of sense as its the Tim Burton Superman) somehow captured by the Soviets when he was a young kid (with the help of Lex Luthor, played by say Johnny Depp in flashback, further tying into the Tim Burton-verse), held prisoner for like 25 years (since his film was supposed to come out, he'd look older due to the effects of long term kryptonite exposure).. Now all unsure unsteady neurotic and long haired and suddenly having to take on this huge threat of Zod with Batman and Flash.. Then you'd have Keaton Batman and Cage Superman basically headlining the movie and bouncing off one another.. also unlike SG in the movie you could have Cage Superman (rejuvenated and deaged slightly) actually succeed in defeating Zod (with Batman's help).. But its like a hollow victory as the timeline/alt reality then comes crashing down due to alt Barry, and 'prime' Barry has to go back to the speedforce to try fix everything (leading to all the cameos etc)
Keaton Batman with Cage Superman probably would made alot more box office..