Banshee: Sean Cassidy in X-Men (introduced as part of the second gen X-Men in Giant Size X-Men 1 along with Wolverine, Colossus, Nightcrawler, Warpath and Storm). Banshee lived on in the comic books for some time, a stable, reliable side character and came into his own as leader of Generation X (alongside Emma Frost) in which he, Frost and Sabretooth kept the next generation of kids out of the way of the Phalanx. In X-Men: New Class he’s almost the only character placed pretty much in line with continuity in the books as his age in the sixties reflects the age he’d be now (relative to Prof X). A mainstay of the books for some time Banshee was killed trying to stop a plane crash.
Havok has occupied most of the theories surrounding the decision to start at the beginning. Havok’s most notable feature in X-Men is that he is brother to Scott Summers (Cyclops) and has always lived slightly in the shadow of his brother. That clearly isn’t the case here as Cyclops is part of the modern day canon of the previous movies. At the X-Men: First Class panel at MCM it was a question aimed squarely at the writers. All they could say was that Havok is related in someway to Scott. Hard to figure out where they’re going with this but if a franchise is built hopefully all will be revealed.
The inclusion of Mystique is an interesting choice. Most viewers of previous X-Movies’d be aware that Mystique has a very clear resolution to her plot line. Her association with Erik Lensherr in this adds a neat reference point to both sets of movies (with the Wolverine Origins franchise sandwiched in the middle).