I would hope to see an extended space battle. Characters like Superman and Green Lantern working together in space. It could also hopefully show off cool GL effects.
As said, a cooler version of GL in general. Maybe even brief scenes of Oa, the Guardians, the Corps etc.
I would like to see Wonder Woman engage in a very physical battle, with armour, sword, shield, lasso etc.
Batman should have more hi-tech gadgets, and just use them in ways we haven't seen yet, to really display the potential of Batman onscreen beyond Nolan's movies.
It would be cool to see the electricity and sparks flying off Flash when he runs. And really show the appeal of his character through not just the effects, but his personality.
If Aquaman is shown, the first shot of the city of Atlantis should be awesome.
If Martian Manhunter is shown, his transformations would be cool, as well as the sadness/pathos of his character, his isolation on Earth. But, like the JL stories we have seen, his acceptance into the League helping him.