Desires for FILM RED:
1) Shanks is, was, and always will be "just some guy". No secret Celestial Dragon lineage, no secret D. name, none of that shit. Just a regular-ass dude.
2) This is a side-story that has nothing to do with the Strawhats, sort of like that Ace novel that Boichi drew.
I have a really hard time thinking 2 is even possible, because of editorial mandates to have the main character X% present in any given adaptation. Which if Luffy has to be in the movie- barring some sort of dueling-narrative structure- leaves only two real possibilities: it's before or after Shanks leaves Foosha for the last time.
It's also possible that 2 can technically happen, as long as there are numerous scenes where Luffy is on screen. Like I said, could be a dueling-narrative structure, or simply have Shanks have multiple flashbacks to Luffy or reading about him or whatever. Might even have been spying on his progress at certain times in the story that we don't know about.