I think folks should refrain from getting their hopes up about this Superman x Batman union in the MoS sequel. It'll likely just amount to an extended cameo by Bruce Wayne.
This is how I imagine it will play out:Warning: Spoiler! (Click to show)
In the aftermath of Metropolis' destruction, we are witness to a city attempting to repair itself. Taking part in the rebuilding efforts are several prominent citizens and home-grown companies, of which Lex Luthor and his Luther Corp. are a part of. Also in the mix is Wayne Enterprises, whose billionaire CEO has pledged some undisclosed amount (speculated in the millions) to the "affected citizens" fund.
In proper fashion, a huge and ritzy gala is thrown in recognition of those people who've done the most for Metropolis in the rebuilding efforts; and of course, as is always the case with such events, much of the attendees hail from the upper echelon of the social order, with nary a consideration/invite for the men and women actually involved in the build up effort (see construction workers, reg. on duty cop, firefighters, nurses, etc). Simply stated, the gala is an ensemble of the rich, for the rich, to speculate and amuse themselves (who lost the most properties, whose insurance policy paid out the most, etc).
Anyway, as it is customary to document these events for the hoi polloi, certain privileged members of the press are given access, and as we scan the room we stumble across Perry White and Lois Lane from the Daily Planet, naturally, and Clark Kent, quite unnaturally. Clark has been dragged there in lieu of a more seasoned journalist because Lois believes the event will hone his journalistic skills (throw him to the sharks and he'll have no choice but to swim for his life). Also, having Clark there frees her from her journalistic obligations and allows her to cavort with the rich.
After a few overwhelming profiles, Clark retreats to the food spread at the back of the room to take a breather. There, he surveys the room, full of people and pretentions unlike anything he's experienced. The contrast is stark for a guy who grew up on a farm in Kansas. He continues to reflect on the simple life he once knew, but his longings are suddenly interrupted by a voice next to him. A voice that seeming crept out the darkness like a phantom. There's a sharply dressed man standing next to him, holding a drink in one hand, with the other hand tucked in his trouser pocket. Clark wonders to himself, "when and where did he come from"?
A man whose presence escapes detection by even the Man of Steel, remarkable, wouldn't you say?
The two chatter for a while. It's obvious to Clark that the fellow is cut from the same social cloth as many in the room. However, beneath the superficiality of wealth, there appears to be more to the fellow. A great deal more. Clark is mystified. Their conversation is interrupted by the shriek of a microphone on the other side of the room. The room goes quite.
A man who was earlier introduced to Clark by Lois as Lex Luthor, "one of Metropolis' shining stars", holds the mic. He makes a few opening comments, jokes, the crowd laughs pleasantly. He then acknowledges some of the various esteemed invitees in attendance--there is applause. After the applause dies down, he starts addressing why they are all there. He talks about taking a stand, a need for action and a call to action to protect the world from a new kind of threat. He talks about the rampancy of chaos in the world. Of mad terrorists hell bent on destroying cities, of aliens hell bent on destroying earth, etc. He talks about brave men and women standing for something. Of heroes arising from the ranks of the ordinary to stand up for justice. He touches on the vigilante known as the Batman in Metropolis' sister city, Gotham. There is faint but recognizably haughty murmur in the room. He talks about the "good alien" who protected the city. There is an approving murmur in the room. Still, he continues, these "heroes" are outliers that cannot be counted on or be trusted. He talks about taking matters into their own hands--there is approval in the audience. That in conjunction with their defenselessness in the face of considerable local and global threat, he explains, is the impetus behind his forthcoming revelation--a joint R&D partnership between his company, Luthor Corp. and Wayne Enterprises, with backing from the U.S military, aimed at shoring up Earth's defenses--on the micro and macro level--in the face of increasingly intimidating forces. There is great approval in the room, then clapping.
Lex, looking satisfied, reveals that there is more to be said about the matter, and invites a special guest to join him: "Bruce Wayne, get out over here...."
There is restlessness in the gathering as everyone looks about them. From various pockets, the curiosity is audible: "Bruce Wayne, here...well isn't that something..."; "I heard he lost his fortune in some scandal involving stocks..."; "He never called me back after that night, I thought we really had something special...."
The anxiety in the room is palpable. Even Clark, at this point, is wondering who this Bruce Wayne fellow is.
Lex scans the ball room, stopping and fixing his gaze in the back, to where Clark is standing: "ahh, there he is; Bruce Wayne you rascal, come on up here."
Everyone in the room follows Lex's gaze towards Clark. Clark looks at all eyes seemingly on him like a deer in headlights.
The guy standing next to him, who he was briefly chatting up right before Lex's interruption, has set his drink down. The fellow looks Clark in the eye and explains how he hates these things--pointing to Lex holding the mic. "Well, guess being the CEO can't be fun all the time, how's my tie by the way." His tie is set and held sharply in place. Clark echoes, "looks great," still faintly perplexed but now approaching the shores of realization as to who this fellow is.
He thanks Clark and pats him on the shoulder, telling him he's a decent fellow and that he enjoyed talking to him. He begins walking toward Lex on the other side of the room. He walks calmly and confidently like a man who has rehearsed this many times before. Like he were an actor and this was all an act to him. "Bruce Wayne everyone...," Lex announces once more into the mic, with applause all around.
Clark looks at the frame of the man walking to the front, musing to himself, "Bruce Wayne..."