Sounds strange that they're offering the role to pretty much any male 40-50 when Mister Sinister is the most British supervillain of all time. I can maybe see Idris Elba playing the role but only if he's allowed to use his normal British accent and even then, there has to be the question of why/how a black man was able to accumulate enough wealth to run experiments in Victorian London. One line if dialog changing to state that it was hard for him to become a scientist due to people's prejudices.
What I don't want to see is an American Mister Sinister. Somebody like Matthew McConaughey or Tom Cruise not even bothering with the accent would be the most horrible casting choice that Fox could ever possibly make.
My Sinister shortlist is
Daniel Craig
Idris Elba
Hugh Grant
John Barrowman
Peter Capaldi
Gary Oldman
Hugh Laurie
Christian Bale
Colin Firth
Ewan McGregor