really you need to ask... What drove Eric Lenshire to become MAGNETO... and build the characters around that. I think if you look at it in groups.
> His distaste for humanity...
His parents being killed, Magda being killed but revealing Magneto has children, Quicksilver and Scarlet Witch. Which he knew nothing about because of what happend to Magda. Magneto and Xavier first working together during the war, possibly would make sense for Magneto to see the full force of Xaviers onslaught use of power.
> Then there is his brotherhood in the making...
Mystique, Sabretooth, Toad, Possibly his first encounter with Jason Stryker, Avalanche would be a new character to the mix. Maybe blob can be explained for briefly appearing.
> Then there is Magnetos plan to turn everyone into mutants witch he isnt a scientist so he had to have gotten that from someone.
Best bet would be to include The High Evolutionary because of his work of turning humans into mutants. and vice versa, We could say the High Evolutionary is the inspiration that Magneto needed.
So in total...
Magda
Quicksilver
Scarlet Witch
Young Xavier
Possibly, Nitro - as a weapon the enemy have captured and exploit his power which promts magneto and xavier to work together.
Blob
Mystique
Sabretooth - transformation from the two films can be explained by high evolutionary
Toad
Avalanche
Jason Stryker
and
High Evolutionary.
Oh and to add one on the end, Destiney