Depp may not want it, first. And secondly, based on Marvel Studios' past behavior with the exception of RDJ, they seem to let folks walk rather than pay them the big bucks. Depp would have a very high salary to start with, that could potentially get even higher if his Strange caught on with fans. I also think he could do a great job, but realistically would expect a "poor man's" Johnny Depp like a Jared Leto