One of my random thoughts that I actually acted upon. When you absorb a dragon's soul and spend it, what happens to it? I mean they probably don't go to the soulcairn, and (correct me if I'm wrong) they definitely don't go to Sovnguard. So what do you guys think happens to them?
I believe they stay within the Dragonborn, eventually returning to aetheius after the Dovahkiin dies. There are texts which say that people couldn't look upon Ysmir without seeing a dragon. I propose that the more souls a DB consumes, the more dragon-like he becomes. Paarthurnax says something about the will to dominate. I think this urge gets stronger the more dragon-like you become.
It's all open to interpretation but yeah, that's how'd I'd put it, minus the split apart bit.
It depends on how you view the dragons. Are they individual spirits? Or are they all just pieces of Akatosh? Alduin claims to be "first born of Akatosh" but we can't take that too literally - or maybe we can.
Since there is only speculation, I'll speculate.
Their souls merge with the Dragonborn's, and stay merged even if the Dragonborn dies (and doesn't get absorbed by a dragon(born).) There's a sort of meta theme of souls ascending beyond the confines of the Wheel, and one attempt at doing so, pioneered by the Dwemer, is to merge enough souls back together as to kickback into a previous, or higher, plane of existance.
I think M'aiq, while breaking the fourth wall, is telling the truth. "Dragons were never gone. They were just invisible, and very very quiet." They didn't leave Mundus (they're et'Ada, they can't), but as just their souls and bones, they couldn't shout or be seen.
Well, I believe that the reason Miraak went evil is due to living in such close proximity to dragons. When he became envious of his dragon masters he turned to hermaus mora, who gave him the soul of a dragon. As he continued killing dragons and absorbing more of them he became more power hungry. Exhibit A being modern Miraak. Though half of that may just be my insane ramblings.