"We talked about a role. We had a conversation. I’m pretty sure I was busy doing something else in the summer he was kickstarting that."
I recall thinking at the time that he would have made for a great villain, although we didn't know anything about Kylo Ren at the time. Adam Driver won that role and was perfect for it, so no loss there. Maybe Fassbender was up for Oscar Isaac's Poe Dameron?
Anyway, it doesn't matter now, and with Fassbender hoping for an Assassin's Creed trilogy he may be too busy for future Star Wars movies.