So we all know till now Lex Luthor will be the villain in Batman vs Superman, also in Man of Steel sequels, not confirmed. And Jared Leto's tattoed joker will be there in Ben Affleck's solo Batman movies (not confirmed on this either). But imagine, this two TOGETHER!! Joker+Lex=Chaos! We fans, would definitely love to watch them working together, in killing their counterpart.
Lex is arch nemesis of Superman. Unlike Supes, he doesn't have super strength, bullet proof body, heat vision, ect. But he possess high intellectual, disdainful of physical confrontations,constantly damages and always finds way to fight Superman. Also he is the polar opposite of Superman. To fight Superman he has huge stock of Kryptonite and he has developed Power Suit. This traits make him more powerful than other Superman enemies.
The Joker, whatever you may call him, psychopath, killer, criminal blah blah blah...., he is the top villain in pop culture. What makes Joker dangerous is his versatility and fickleness. The Joker is obsessed with Batman, they're like Ying and Yang. Joker wants to prove that even Batman is same like him, a freak, and to prove his point he'll go to extreme lengths, even willing to die. Joker stands for what Batman opposes, a perfect nemesis to perfect hero.
We've seen this two characters coming together in animated movie Batman and Superman: World's Finest. Joker attacked Metropolis and tried killing Superman with Lex's help, but ended up killing himself. Also in Justice League Animated Series we've seen how much of a pain in the ass they're to JLA. So they should crossover at least once. If they come together it means again Bats and Supes team up, because either one against them would be ultimate suicide. Hell we might need the whole Justice League(yes trying to give ideas for Justice League movie). Lot of action! Plus a good story line. It would be delight for fans! On the other hand it would be prove as something different, that MCU hasn't come up with. Sort of a challenge. To compete with Marvel, DC needs to this different. Different in the sense of arch enemies, of top two superheroes of DC, working together, plotting world domination or helping each other to take down each other's counter part.
Such movie would be awesome and pure joy to watch.