X-Men Origins Magneto

X-Men Origins: Magneto is a cancelled X-Men sequel that was supposed to come out after X-Men Origins: Wolverine.

Plot
The plot is currently unknown, but it would have focus on the origins of how Erik Lehnsherr became the supervillain.

Why It Was Cancelled

 * X-Men Origins: Wolverine received mixed reviews from critics, but panned by fans for its bad script and poor characterizations of the characters, especially Deadpool.

Result

 * Plans for the Origins sub-series where cancelled.
 * X-Men Origins: Wolverine became part of the X-Men movie universe instead.
 * 20th Century Fox would end up releasing X-Men: First Class and The Wolverine.