Assuming that the current films stayed the same (meaning the extra films can't use villains already used), and trying to be somewhat realistic about the output...
A third Burton/Keaton film
Batman: Fear
New characters: Scarecrow played by Jeff Goldblum, Black Mask played by Christopher Walken, Firefly (only a small role), Sasha Bordeaux played by Bridget Fonda, Lucius Fox (another small role)
Returning characters: Alfred, Max Shreck (as Black Mask), Gordon
Scenario: Batman has consumed Bruce's life, leaving his business world to crumble. Wayne Enterprises has employed Jonathan Crane to develop fear-inducing toxins for the military. Meanwhile Max Shreck from the previous film has survived to become Black Mask. Bruce is forced to take on a bodyguard (Sasha) by his board at Waynecorp. after Firefly (hired by Shreck) attempts to kill him in broad daylight. Thematically, this would be about heat and fear, with Gotham sweltering in summer and Black Mask terrorising the city at the same time as Scarecrow.
(I actually wrote a spec. script for this one once...)
A second Schumacher/Kilmer film
Batman: Dark Knight
New characters: Harley Quinn played by Jerry Hall, Mad Hatter played by Steve Buscemi
Returning characters: Robin, Alfred, Gordon, Chase Meridian, Alicia Hunt (from '89, as Harley Quinn)
Scenario: Alicia Hunt, Joker's 'art project' from the first film returns as Harley Quinn, with a vendetta against Batman. On a quest to cause insanity, she kidnaps, tortures and breaks Jervis Tetch, a WayneTech employee, turning him into the sadistic Mad Hatter (sounds a little similar to the Riddler, I know, but hey, this is meant to be Schumacher after all). Thematically this would be about the demons of the past. It would also give us a better reason for Chase Meridian leaving.
A second Schumacher/Clooney film
Batman Triumphant
New characters: Robin becomes Nightwing, Man-Bat (small role), Killer Croc (small role), Charaxes (small role), Ra's Al Ghul played by Mads Mikkelsen, Talia played by Famke Janssen
Returning characters: Batgirl, Alfred, Gordon, Robin (as Nightwing)
Scenario: After Batman & Robin, an almost Batman Incorporated scenario has been developed, with Batgirl and Nightwing as 'agents'. This would be a globe-trotting entry with Batman stationed in Paris, Batgirl at home in Gotham and Nightwing in the Middle East (with them coming together at Gotham at the end). Nightwing is tracing the mysterious head of the League of Shadows in the Middle East, after several earlier attacks on Gotham City. When they begin mutating citizens of Gotham into 'splicers' such as Man-Bat and Killer Croc, Batgirl has to keep the homefront under control. Meanwhile Batman is trying to find Talia in Paris, hoping that Ra's al Ghul's estranged daughter will give them the help they need to stop The Demon's Head from destroying Gotham with splicers. This would really be action-oriented and very adventure-driven.
A fourth Nolan/Bale film
Shadow of the Bat (set BETWEEN Batman Begins and The Dark Knight)
New characters: Huntress/Helena Bertinelli, Penguin played by Jim Broadbent
Returning characters: Alfred, Lucius, Harvey Dent (cameo), Gordon, Rachel, Scarecrow, Sal Maroni
Scenario: Set between the first two films, this shows the clean-up of The Narrows. It also shows Scarecrow as the primary, big-bad villain, with a rogue Huntress threatening to kill the mafia bosses as a secondary villain. Meanwhile the Penguin rises as an arms dealer. In the background there are references to escalation and the 'Joker'. The themes would be 'inspiration' and 'escalation' - what happens when the symbol you've created inspires violence?
It's fun saying 'what if'.
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