How to bring back Will Smith for the third one (if he wants to)
It was established that his character died testing one of the first alien-aircraft prototypes.
However they never said a body was found.
If Will agrees to return to save the franchise, they might just say that during the "test", Steven Hiller was attacked by an alien who had survived human's victory, crashed his aircraft, survived and then was abducted and taken to the main enemy ship that we might see in the third one. Then, Dylan can go and rescue daddy.
Then the characters can say "well, we never found the body, we presumed he was dead".
It's cheap, but I've seen cheaper.
I once told a man to go screw himself! Can you even imagine?! - Kilgrave