Today I am going to present another theory to you all, this one is about (surprise, surprise) Alduin. It is going to be a reasonably simple but interesting theory.
My theory is that Alduin is Konahrik, the Warlord.
My reasons for thinking this are many:
Akatosh (Alduin’s alleged father) has been depicted as a man, a dragon and an elf (Auri-El).
Alduin himself has been depicted as a man wearing Dragon priest garbs in ancient Nordic sites.
The mask is hidden and protected by a bust of a dragon head.
So if Akatosh could change his form from dragon to man, it is not far-fetched to believe that Alduin can do the same. There is, of course, even more evidence to support this theory and that evidence comes from to places: Alduin's Wall and Bromjunaar Sanctuary. The Wall shows Alduin returning to battle the Last Dragonborn after being sent forward in time and the only way to get his mask is to go back in time to when the Sanctuary was still intact.
This to me means that the remaining priests used ancient and powerful time magic (something that Alduin, Son of Akatosh could have very well known and have taught his followers) to enchant the wooden mask with the power to send the user back to just after they stored away the mask. They must have been confident enough to believe that Alduin would kill the Last Dragonborn allowing the Dragon Priests to bring the mask forward in time.
In short, Alduin is also a man with a mask keep locked away in place a few thousand years ago.