Yes.
Esplain
First tell me who the big three are; Superman, Batman and ?
Wonder Woman
Wonder Woman? Really? Whatever.
Anyway, I think characters with super powers wouldn't fit the dynamic of the show. Batman, might work, but the other two would come off looking like cheese balls dropped on party trays just in time for sweeps week.
I think an appearance by Bruce Wayne would be fantastic.
Imagine, Wayne is visiting Starling City (maybe a Queen/Wayne business transaction) and Ollie uncovers a kidnapping plot. He intervenes only to find Wayne is more than he seems
Just a thought, I'll be here waiting for my royalties