Imagine this...
If they have Scott, Jean and Ororo as say late-teenagers, have Beast in there as well (even though he'd be almost 40 years old), and then have Warren Worthington in as well (there are three ways to do this, two are the wrong way, and one is the correct way), then we'd have a movie with the 5 original members in the same timeline.
To get Angel they would need to do one of these three things:
1. Have him transported back in time from the future. This is a wrong way as we've already had enough time travel stuff.
2. Have him born 20 years earlier than he should be. This is also the wrong way since it's not feasible. Either he'll be born when he was in the original timeline, or not. His birth would be around 1983 or so.
3. Have him taken as a baby by Apocalypse or one of Apocalypse's minions (Sinister, Gambit or Mystique) and have him go through the Horseman altering process, which includes aging him. We'd see his feathery wings grown in and then see them being amputated. The process would end with him as Archangel.
But the point is that at the end of the movie we'd actually have the five original members in a position where they could actually have their own movie...