Hello!
Let me first say, that this "theory" of mine is probably wrong. It's just an Idea I had while I played Skyrim.
Clavicus Vile says in the Quest "A Daedra's Best Friend" that the LDB is as powerful as him, because he doesn't have Barbas at his site. Now, let's just say, that Clavicus Vile's power + Barbas power would equal 1. The power of Clavicus Vile alone would equal 0.5 (Barbas power too). The Last Dragonborns power would then also eqaul 0.5.
But there's also Miraak. Miraak wants to break free from Hermaeus Mora, he wants to flee from Apocrypha, and he says (as far as i remember) that he needs the LDB's soul in order to be able to become free. Miraaks power would equal 0.5, like the LDB and Clavicus Vile. Everyone knows that the LDB absorbs at the end Miraak's soul, and my theory is that the LDB's soul (0.5) + Miraak's soul (0.5) equals 1, or the power of Clavicus Vile and Barbas. Does this mean, that the Playerborn becomes as powerful as a Daedric Prince? In my opinion, yes. Or at least, maybe.
There is of course one thing that speaks against my theory: That the LDB's and Miraak's soul are "just" Dragon souls, which would mean, that Miraak could have escaped since a looooooong time from Apocrypha, by simply absorbing a Dragon's soul. But this is NOT the case. Miraak can appear for a short time in Nirn (which is the case when he "steals" a Dragon soul from the LDB), but only for a short time (and he seems somewhat transparent too).
This means that there is maybe something special about a Dragonborn's soul, something that makes it different from a normal Dragon soul. Is this possible?