Well, aside from the obvious ones like Superman and Batman, Martian Manhunter is a great character to include. His alien nature and diverse powers such as shape - shifting, invisibility and telepathy can add a whole new layer to a crossover. Then there's Cyborg. His half - human, half - machine nature can create some really interesting interactions with other heroes, especially those with more traditional powers. And Harley Quinn, even though she's more of a villain, her chaotic and unpredictable nature can make for some really fun crossovers with the heroes.