User blog comment:Jack Pistol/Immortal Dovahkiin?/@comment-3534737-20121003015124/@comment-1080767-20121004051251

You don't need the other Dragon's soul to resurrect it, you just need it's body, because it's soul is still connected to the body. I think it's reasonable to assume that Alduin didn't absorb the souls of fallen dragons - the dragons that he resurrects throughout Skyrim are able to be resurrected because their souls are still there with their bodies, since they haven't been stripped away by a Dragon(born).

So, as I said in an earlier post, there could be a DLC in which the player sets off to learn a shout that does what Alduin does: resurrects dragons. The player can use this shout to, say, revive skeletal dragons like the one in Labyrinthian. Anyway, the Dragonborn then goes and locates the tomb of Tiber Septim himself, whom the Dragonborn proceeds to revive. There could be multiple reasons for which the Dragonborn would be motivated to do this: figure out if he really became a god, lay claim to the crumbling, desperate Empire, fight against the Thalmor or other threat that may be too big for one Dragonborn, etc. Some time after the restoration of the Septim bloodline, Tiber becomes corrupted somehow (maybe to prevent an all-out war with the Thalmor that rejected his divinity; surely he would be pretty pissed about that). Ergo, the player would have no choice but to engage Tiber Septim, and upon his defeat, his soul is absorbed, entailing perchance the acquisition of a god-like status, or, if not, Tiber Septim becomes a true god (not that he was or wasn't earlier), his soul now channeled through the Dragonborn, who is alive and connected directly to the Divines. I'm sure the Greybeards could fit in here somewhere, as well as the Blades, as Jack mentioned.

What would be cool is that if Tiber Septim is used as a follower, the player can (re)visit Word Walls of uncommon shouts that he probably never learned and Tiber will learn them. Moreover, if the Dragonborn 'dies' with him as a follower, they may be revived by him through the appropriate shout (overpowered, I know, but if this were an actual DLC, my justification for this is that Tiber wouldn't be used as a follower for most of the questline.