I can think of a few ideas...
A Kingdom come type storyline : Michael Keaton back as Batman/Bruce Wayne with a bit of Batman Beyond thrown in. Diana, Arthur and Clark have hardly aged. They face Gog, Magog and a bunch of less than worthy violent heroes...
Justice League/Suicide Squad crossover. Similar to the 2017 "event" with Maxwell Lord and Lobo as a breakout character. Since JL made less than hoped and SS more than expected, this might make dollars and sense...
Doomsday Clock. A sorta sequel to Watchmen with the Adrian Veidt and Dr Manhattan actors returning. (easier said than done I know)...
and YOUR ideas?