Dragon Priest Mask theory
I don't konw if this theory existed beforehand, but I had an epiphany the other day watching Epic Nate's Skyrim mystery iceberg. Because:
What if Konahrik mask, the only dragon priest mask without one priest, is due a time Paradox?!
If we analyze the known fact:
For us, and the original priest, it's needed all nine other masks together to open the "safe". We could say would take time for all nine to meet together to get it. (I don't remeber if it's mentioned how those priest are chosen)
And the most important point is:
If we, the player, took the mask in the 4th Era, during the gameplay, this means there would be no mask for the intent priest in the Merethic Era.
So, the "maskless" dragon priest happened because there were no Mask to be take when it was meant to be, since that mask was now in another place in time.