Well, they should! It would be awesome.
Totally agree, just make something that explores the entire breadth of the DC Universe.
Begin on earth, maybe an incursion into a Cadmus laboratory where a very nasty discovery is made that changes everything (A Black Lantern Ring along with some Apokolips tech for example). Switch to an urgent meeting on the Watchtower and then a cut-scene showing villains like Joker, Lex, Sinestro, Despero meeting together and hatching some sort of plan to steal those objects.
This could open up arenas like Oa, Apokolips, Metropolis, Gotham, although anywhere would be open season
And of course bring on the characters and cameos...