In a recent interview with MTV Kevin Feige said that Ant-Man will officially start Phase 3 of the MCU, and when more new characters were brought up he said
"'Doctor Strange,' which I've been talking about for years, is definitely one of them," he confirmed. "He's a great, original character, and he checks the box off this criteria that I have: he's totally different from anything else we have, just like 'Guardians of the Galaxy.' He's totally different from anything we've done before, as is 'Ant-Man,' which keeps us excited."