Letting casting difficulties disclude War Machine was a big mistake. Shoehorning him into the story would have been challenging, sure, but not impossible... unless, of course, they're saving him for the sequel. That, I'm all for.
(I don't blame Marvel or Whedon for the lack of diversity. The core heroes of the Avengers are from an era in which the best-known superheroes were Big White Men. I'm sure it was considered "diverse" enough to have heroes that were heroic by way of Smarts rather than Strength (though, invariably, they just used their smarts to get strength, so...))
Unfortunately, I think a lot of the go-to "minority heroes" play to far too many stereotypes. Black-guy-as-written-by-70's-white-guy, or Asian-guy-as-written-by-White-Bruce-Lee-fan.
I say this in all honesty: I would rather see them go with a minority Dr. Strange. And not a "voodoo" remake to shoehorn in "He's Black!" Dr. Strange. But a black (or Mexican, or 'Middle Eastern,' there's a lot of ground to cover) guy.
He's a character that brings a new flavor to the mix (arcane magic, as opposed to science, technology, sheer will, or godly magic). He's a character that many folks are slightly familiar with, but maybe haven't really latched onto before, which makes him a good place to make some changes. He has depth and backstory, he's intelligent, he's really a great character to include... and there's no reason he has to stay white.
What's more, this creates a character that isn't a token minority, at least in the way I view the concept. Yes, in a sense, it's changing the race of a character simply to have a change of race... but it's better than creating a character designed specifically around how "minority" they are.