It's still a little weird. This really seems like it needs an object....Who's opening the door? He opens the restaurant. It would only be used like this in referring to shifts at a restaurant or shop. Who is opening tomorrow?
Or being the opening act of a music concert or live comedy show. "He's opening." as a complete sentence is common in that context.
It also might mean "He opens up," so he himself is kind of the object, as in "He opens himself." It obviously needs the particle to make sense in English but maybe doesn't in Swedish? Just brainstorming.
He opens up (like in, 'he starts to share things about himself') would be han öppnar sig in Swedish.
Is this like how you would say "come in(side)" or "whose's there?" or something? I don't really understand
I don’t know in which context this could be used, but I could imagine several contexts. The example I gave above was a context I imagined where someone knocks on the door and there are several people inside and one of them asks who should open the door and say vem öppnar?.
So is this semantically locked into an antipassive interpretation or might it also be understood as reflexive or passive in the right circumstances?
In other words, would the sentences "The door opens." or "The box opens (itself)." use the verb öppnar or would they use different verbs/verb forms? The same question applies for all intransitive clauses.
It could absolutely be used to mean e.g. "He opens [the door]". Actually, I'd default to assuming that.