I think this is a great idea, though as a player interested in lore based immersion, I believe it would be better for the character to be able to travel to the past, and not the future. I don't particularly want to kill enemies in skyrim with a gun, for the shooter side of my personality I play fallout. Also I think it would be interesting to start the quest by being approached by the otherwise useless headless horseman after the player reaches a set level, and is given a note written in the dragon language, then after receiving the note getting it translated by either a dragon, i.e. Parthurnax, or Odahviing, or a mage at the college of winterhold. Then finding a few new items so the player can enchant the wooden mask to be able to speak to, and understand the old nords that speak the dragon language.Then meeting the headless horseman at Hamvir's Rest, and being instructed to put on the mask to investigate and prevent an untimely murder in the past, that was committed by this cult of mages, and in the process learn their secrets so you can traverse time at will by either equipping/unequipping the wooden mask, using a new power/shout, or using spells. Furthermore, though I have no modding experience I believe it could be done similar to the process done here only with the existing skyrim world places while deleting the more "modern" towns like Riverwood. Though again, I am not a modder, and the only experience I have on the matter is playing with Unity3d to work on building my own game.
