Dum Dum Dugan can't really be a character since Captain America was sleeping for 70 years in this canon (if he's not dead he's ancient now). I only recently saw it too and it's a pretty fantastic movie. I think we could have used a SHIELD movie or something that just showed Nick, Black Widow, and Hawkeye doing things, but that probably wouldn't have the same appeal to casual movie goers as the other Avengers have. My only real complaint was that the Chitauri could have used a more present leader since Loki wasn't officially their commander or anything (though I know nothing about them in the comics, so it may not have made much sense anyway and inventing a character always serves to piss off hardcore fans).
The Chitauri have a leader in the comics, but he would have been too similar to Red Skull - the Chitauri are shapeshifters, and their leader has been a Nazi since the second World War.
As for Dum Dum Dugan, well, there is a reason I want more of him, and that is that he, along with Gabe Jones, Nick Fury, and a number of others, are all actually very old, but were given the Infinity Formula, which has drastically slowed (if not halted) the aging process