It would have been tough to kick off the movie with an action scene (a modern standard). And it would have been harder to present the time pressure that they were under. Although, Magneto going rogue and potentially ruining the future himself is kind of pressure enough.

Really, the movie should have been Mystique's. She's the one driving all of the action and pushing the story forward. It could have been a revenge film questioning the impact of revenge. Again, probably more of a winter movie than a summer blockbuster.