Speaking to Comic Book Movie, Johnston said that he would consider making a follow-up to the superhero film if the storyline was compelling.
"I would be interested if the story is as unique as Captain America. That's a tall order, but if Markus and McFeely are writing it there's a very good chance it will be great," he told the website.
"There are other interesting characters in the Marvel Universe, for instance, the Winter Soldier, aka Sergeant James Buchanan Barnes."
Captain America stars Chris Evans as the titular hero. The actor recently admitted that he attended therapy before the project commenced, and endured "brutal" fitness regimes to get into shape for the role.
Captain America: The First Avenger arrives in theatres on on July 22 in the US and July 29 in the UK.
Watch a trailer for the movie below: