You'll See Your Problems Multiplied: Five Reasons Steve McQueen's Next Film Should Be a Comedy
5. Pitt can be (kinda) funny.
4. Fassbender can sing!
3. Fassbender can dance!
2. Fassbender wants (and needs) to work on his comedy skills.
1. 12 Years a Slave is almost too depressing for words--I want to cry just reading about it: In 1841, Northup was made an employment offer by two men, to join a circus as a violin player; the scammers lured him to a hotel room, offering to pay his board there. When Northup showed up he was drugged and along with others, sold into slavery. While in captivity, Northup was beaten and whipped, passed to different owners and nearly killed (with an ax) by one. Inasmuch as this story should be told, perhaps you could consider passing it along to another director.
Since you are already committed to your stars (and they to you), why not instead make a comedy (maybe even a musical) and let Fassbender sing and dance? You could call it 3 Years a Moron: Life as a Circus Clown. More people will come, everyone will be laughing and happy and you'll make more money. And isn't that what this industry is all about?
Cindy Davis just wants us all to get along.