Whether it leads to legendaries or not, the doors alone can't give all the legendaries, we'll probably have new maps, i think it'd be kinda stupid to get legendaries like zapdos, articuno and moltres from these doors, they should be in some islands like they are in the story, having only two locations with the boat is kinda stupid right? i think some of the doors, like the one arceus is in and the one under the stairway probably have legendaries, under the stairway i believe there's a whole city AND a legendary, as for the rest, they'll probably lead somewhere else along with more secrets and legendaries.
I just hope that we'll manage to finish this game while we still live.