Nomad in the comics was from one of the other times he gave up being Cap, back in the '70s when the POTUS of that universe turned out to be the leader of a terrorist organization. He resigned his commission, as it were. That was back when nobody would have even thought to make a black man Captain America, so the mantle just went unfilled for a few issues.

As much as I like Steve and Chris Evans' portrayal, I wish they weren't bringing him back. Or at least, not as such a focus. If Doom is the main antagonist, then the main protagonist needs to be Reed, full stop.