As cliche as it is, I'd love the Worf effect for Darkseid. Tears apart Mongul or Brainiac after they beat the league (though I want the league to have some symbolic victory). But yeah Mongul and Darkseid are very familiar so I have a hard time seeing them using both without one of them changing a bit. They'll probably budge on Mongul more.
Ok, so I gotta post my Brainiac idea, 'cause I honestly think it'd be perfect for this:
Make Brainiac a Von Neumann probe. There's not one of him, there's
millions. The league puts one of him down after a hell of a fight, and while he's going he brags about how the rest of him is converging on a world whose technology will allow him to ascend to godhood, that he'll be back, and this victory is purely temporary. Cut to a fleet of Brainiac ships surrounding a planet with giant columns of fire spewing from it. Suddenly, a cloud seems to emerge from them, from every pore of the planet, and as it grows it becomes clear that it's not a cloud, it's billions upon billions of winged...
things. Battle is joined, and several Brainiac ships are torn apart in moments. They take a terrible toll, but the Parademons just keep coming, endlessly. We finally get a sense of scale as one is brought down, and this world is MASSIVE. A handful of ships escape the swarm, and aboard one we see what amounts to an innter dialogue as the various nodes converse. Suddenly a boom tube opens up, and a massive figure strides into the ship. Defenses that nearly totaled the League open up to no apparent effect. Brainiac begins screaming, and Darkseid... Darkseid
laughs.
Roll credits.