Bring in more than one other and it becomes a Team film not an Ironman film, bring in one and you've either got a buddy cop flick or one is demoted to sidekick and/or moron, just the way it goes. Cameos are usually a bad idea unless you're lining stuff up for another film because it just reminds the audience about the superhero who isn't showing up for the world crisis, and superhero dynamics being what they are it generally invites the thought that the non-present hero would solve the question five seconds flat, or be terribly useless and thus boring for a film. No Hawkeye or Black Widow solo films seem likely

Always a bad idea to add the sidekick to the A-team. If he'd been in there he'd either of had to have a cameo or people would complain that his dynamic with Stark was chewing into time better spent on Stark's interaction with the others, their own interactions or development, or the plot.