I know the others, but who's the guy in black on the far left in the thumbnail?

Black Panther. They hinted at him in Age of Ultron (his home country is the same one where Iron Man fought Hulk in Africa).

I still need to see Age of Ultron!

This looks absolutely horrible!
They will ruin the Civil War story arc.

Is this following a preexisting narrative from the comics?

https://en.wikipedia.org/wiki/Civil_War_%28comics%29
These events are also why Steven Colbert now has the shield.

I don't think they could really present the original storyline in a cinematic format. The comic worked because they were able to employ the ENTIRE Marvel universe, and reference years and years of superhero / supervillain interactions. With a drastically more limited cast and history the story simply wouldn't feel right. They're still using the general concept of the story it appears, but in a significantly more limited context.

I think my only complaint will be that they're not pulling the television characters into CW, but perhaps (hopefully!) they're saving the full-cast spectacle for Infinity War.

And I went to a superbowl party and somehow missed this.

Captain America: Civil War - Trailer 2 has been added as a related post - related requested by OverLord on that post.