When his Supergirl episode aired David Ramsey confirmed that John Diggle’s story with what he found in that box is not over. Here is the direct quote -
https://ew.com/tv/supergirl-kelly-diggle-blind-spots-sneak-peek/
"We just have a moment at the end where we touch on it one more time. This isn't over," Ramsey said. "I think what was important about this and smart from the writers was to tell this story of someone like Diggle, who is such a mentor and such homegrown hero, if you will, that whatever the invitation [in the glowing green box] was — if we remember the story behind Diggle, he had just gotten his wife back, just got Sara back, who disappeared from Flashpoint — that it made sense for him not to accept it immediately, and we wanted to tell that story. This is the last piece of that story. You will see Diggle again after Supergirl, and when you do, we will continue that storytelling."
That's good to know.