I actually bungled his power set.
Originally, his power was pretty much as you describe - he can become a door that his teammates can pass through. He’s kind of a silly hero. But, again, he’s supposed to be. All of the Great Lakes Avengers are supposed to be sillier, less useful versions of existing Marvel characters.
In his case, Doorman originally had a more limited version of the power set of another character, Cloak, who is a living portal to the Darkforce dimension which is…a whole thing in the Marvel Universe. Depending on the creative team and the needs of the plot, Darkforce manipulation can be a pretty powerful power set. Doorman originally could just act as a portal that allowed his teammates to pass through him to the other side of any solid object. But he could also become intangible and grant direct access to the Darkforce dimension.
Later, he developed more elaborate Darkforce manipulation powers, and became a cosmic-level character, and a literal Angel of Death.
And he had a cool costume, in a minimalist sort of way (IMHO).