Mr. Depp could do Doctor Strange, it would require a departure from some of his more recent, eccentric roles however. I look at his performance in "The Ninth Gate" as an arrogant, professional book thief who gets caught up in a search for satanic tomes able to unlock a gate to hell. That isn't exactly the way I'd expect him to play the role, but shows that he has the tools as an actor to not always play so outlandishly oddball.
To be honest also I would make a correction about something. The stuff about Asgard is NOT 100% science fiction, magic is heavily involved there along with advanced science. Loki, The Enchantress, Odin, Karnilla, all of them tap into actual Marvel magical forces. Karnilla is Asgard's "Sorcerer Supreme" each dimension has one, Doctor Strange is ours, she's theirs. So even if one uses the definition that the Asgardians *were* aliens they happen to be magic using aliens, or ones that wound up gaining the ability to use magic. Loki is for example not secretly walking around with a bunch of holo projectors in his gloves, and The Enchantress actually magically enslaves those she kisses, as opposed to relying on some kind of pheromone technology.
That said with Doctor Strange, the big problem with putting Johnny Depp into the role is he's another HUGE name actor who commands massive amounts of money for his time, and is going to compete for billing and screen time with other big name actors. How well he'd play the role isn't the biggest question here IMO, so much as whether having him do Doctor Strange means we'll actually get to see much use of Doctor Strange (who is one of my all time favorite characters).
I would also say that they could have some potential to do a "Defenders" movie like they did "Avengers" with "Doctor Strange" as the launch title, but they already decided to pull that name for "Netflix" ironically filling the team out with "Heroes For Hire" characters (all guys who seem to team with, and be built around Luke Cage, the only one notably missing seems to be Misty Knight as I believe Moviebob mentioned). "The Defenders" actually having started if I remember as being a team created by Doctor Strange to defend the earth against otherworldly threats other heroes weren't dealing with. One of the old school classic storylines was a misunderstanding (well actually several) leading to the "Avengers Defenders War" pitting the two teams against each other... with the predictable outcome of things eventually being sorted out before too much damage was done in the end. Something they could have in theory built a crossover of movie crossovers around.
At any rate I have still been hoping they take the "Defenders" name from Netflix and have them rebrand their crossover "Heroes For Hire" given that it makes the most sense since that's the family of characters they are using.
