So I was thinking about the main quest, having only recently completed it and something really concerning was brought to my attention.

The door to Sovengard, hidden skuldafen is still open.

Think about it, you arrive at the portal which is closed by the dragon priest. He uses a staff to open and close it. After his defeat you use the staff to open the portal and enter Sovangard, but leave without the staff.

That staff, the key to the source of Alduins power, is just sitting there unreachable by mortals, but well within reach of a dragon.

Its not like Alduin is the only one who can enter, Ohdavig even says Alduin guards the temple, why would he need to guard the temple if only he could access it and the only way there was to fly.

Even if a druagr is required to work the staff, a dragon could just command it to do so after flying it there. Which I doubt Alduin would rely on the help of a mortal alone to gain access to his retreat.

That means after Parthunax fails to bring the dragons to the way of the voice(random dragon attacks after MQ means some are resisting) he goes to sovengard to gain more power. Parthanax even says he struggles against his dominating nature, how long before he decides, "a few souls cant hurt." Then his scales become as black as the soul gems that serve the same purpose, devouring mortal souls.

And Parthanax has a long way to fall, who is to say that any other dragon could do the same, with the intent of destroying Parthanax and becoming a new world eater long after the death or disappearance (as most likely with scrolls protagonist) of the dragonborn, don't forget all dragons are immortal and like Alduin says, they can outlast you.


tl;dr?

The key to Sovengard is just laying around right next to its entrance that only a dragon has access to, any dragon could gain entrance to sovengard and gain similar powers to Alduin, long after the dragonsborns death or vanishing act.