Alright here's my hackneyed fan pitch for World's Finest in 2018 following MOS2 in 2016. The only thing that it will need in place from MOS2 is the introduction of Kryptonite and Lex Luthor as essentially the billionaire industrialist/weapons builder.
It starts with Bruce (Christian Bale) back in Gotham City (not publicly) from his retirement as John Blake has gone missing. So he is working covertly at night (not as Batman) stealthily following leads for Blake's disappearance. Luthor (a weapons contractor) is meeting with Lucious Fox for a joint venture in developing mechanized infantry.
Joker is back in town (their will be an aura of mystery and controversy surrounding this both in and outside of the story of course regarding Heath Ledgers death and also in that Joker has been 'dead' in the film world for years). And he meets and is intrigued by Lex Luthor delighted by his hatred of Supeman. He proposes that he can kill Superman whilst keeping Luthors name and image clean. This is seemingly the Joker by the way but at the same time different. Similar face, shorter hair, slightly different mannerisms (all will be explained).
Superman is brought to Gotham and is led into a trap in some industrial district where Luthor has some nuclear-powered proto-mechs waiting. Joker gleefully has a Kryptonite knife and stabs Supes whilst he is recovering from the mechs fire. Bruce who has been led to the area by the leads he's been following is able to get to Superman in time and safely carry him to safety and drive him to one of his underground bunkers.
It's here where Clark and Bruce first talk. Bruce saying how much Clark has changed things etc etc. And he gets the Batman suit to rise from the floor. He witnessed Joker (who still haunts him) during the last scene and that's the only person who could get him to return to the mantle.
The Wayne/Lexcorp deal goes through citing the Metropolis Kryptonian Invasion as the reason for humankind to be greater prepared. It sets the stage for Superman and Batman working together to face these giant mechs (think those mechs in that animated Batman/Superman World's Finest episodes, clearly a main inspiration here).
Superman deals with this mechanized army whilst Batman tries to find the Joker. He does and the big reveal is that this new Joker is actually John Blake who has been drugged and mutilated (this is entirely similar to what happened to Tim Drake in Batman Beyond, and also Dick Grayson in TDKSA, so 2 other precedents for 2 different Robins in the comics). Batman and Joker/Robin/Blake fight with Blake eventually being overwhelmed by his own decaying physical state. We have a scene similar to Death in the Family of Batman holding his 'Robin'.
Ending scene with Superman trying to appeal to and comfort the dismayed Bruce. But all his mental defences come up again and he becomes the Batman we know and love for how he handles pain. The 2 shake and depart separate ways (with a cool takeoff scene of Supes doing his fist to the ground particles rising up and Batman in the Bat with the propellor blades sending smoke and dust everywhere) Superman flies off into the distance where this is a glimmer of sunrise on the horizon. Batman flies in a different direction towards his city back into the night.
WORLD'S FINEST
So what has this accomplished. Well for one it's not a drama-lite Avengers type team-up film but once again emotional and character driven. The most important thing for me is it fixes the ending of TDKR. I'm sorry but Bruce should never have a happy ending. That's whats so great about the character. He is always fighting his own demons. It inevitably never ends for him. But he never stops fighting. That's why we root for him. The ending of TDK is far better because it got this right. (but then the entire sentiment of this was then reversed and ruined in TDKR - 8 year absence what).
About the whole JGL-Ledger thing. Well for 1 JGL and Ledger were buds and I think JGL would think Ledger would have loved for him to follow him up. Plus with the ambiguity surrounding the character and it ultimately revealing to be a new Joker it doesn't mean JGL is necessarily replacing Ledger. Plus how similar they actually look will help with the ambiguity. Just google image search 'joseph gordon levitt heath ledger'.
Directed by Christopher Nolan with visual effects director Zack Snyder.
Obviously I just made most of this up as it's pretty rough. The only things I had in my head was adapting the death of Robin from the comics and Robin turning into the Joker (also from the comics). Plus Cavill's Supes and Bale's Bats interacting in the underground bunker from TDK. I can actually picture this. The realistic tone aimed for from the MOS trailer helps. Plus I think we give the TDK too much credit for being realistic. Let's admit it's pretty comicbooky too. I think it's possible they could merge these franchises.
Anyways rip it to shreds guys!