As for "the black Avengers," I think it's a crap idea. Does that mean no blacks can/will join the Avengers "the white" Avengers anymore?
And then what? The Asian Avengers? The Latin Avengers? The Native Avengers? The Muslim Avengers? Where does it end?
Just diversify the current lineup--and maintain that diversity--and be done with it. That'd be my solution.
Diversity is the best solution in theory. But judging by the latest Avengers lineup, it isn't happening in reality. In a country that's 36% minority, the team is all white.
I don't think anyone is proposing a Black Avengers as a long-term or permanent solution. But as a short-term alternative--perhaps a limited series--I don't have a problem with it. As you may recall, I've proposed doing a Native X-Men story. This would be roughly the same idea.
Global teams of heroes
I had an idea years ago--each nation in the UN had a superhero representative and they all belonged to a global group--a "Justice League Earth", for lack of a better name. Different heroes would be selected for different missions depending on the threat level. Diverse, no?
Has something like that been done before?
If they cared about the Avengers name and not the so-called diversity "agenda," they could use a failed title like the Defenders or Champions for a black superhero team.
DC had its Global Guardians, New Guardians, and Justice League International. Now Batman is creating a Batman Inc. with Batmen (Batmans?) from around the world. These things are roughly the same idea but with a manageable number of characters.
The closest Marvel has come to an international team is its Contest of Champions mini-series. Unless you count all the X-Men characters as one global group of heroes.
"The 99" is an ongoing series about Muslim superheroes. I believe they come from around the world--from the US to Europe to Africa to China--because Muslims live everywhere. The religion is the same but everything else--race, culture, etc.--varies. That's also a similar idea.
For more on the subject, see Comics Diversity Is an "Agenda"? and Top 10 Racist Comic-Book Characters.
Below: The Falcon, an occasional member of the Avengers.