The Dragonborn is clearly Shazzarine.

The Emperor represents Aka

Astrid is perfect witness material.

Now I might just be reaching here, but there seems to be something more to the Dark Brotherhood quest line then what's on the surface.

*edit*

Mind you: This was all carried out in the name of what is speculated to be an aspect of Lunar God.

We already have plenty of Enantiomorph speculation in Skyrim due to the Civil War and Dragonborn questlines. I suppose I wouldn't mind seeing more evidence for this one, but I don't think we really need to paint a new Enantiomorph into the story for the sake of the Dragonborn being a Lunar God; there are already at least two other possible Enantiomorphs to choose from.

That said, the point of Enantiomorph is to betray and replace a king. And I really do want to see a Dragonborn on the throne, if only to hearken back to the glory days of Imperial favor in Akatosh's eyes. If there's a chance that a new Dragonborn Emperor will create a newly-great Empire again (because part of the Enantiomorphic event is the creation of something new, at least as far as I know), then I'm more willing to listen to the theory.

Unfortunately the story of the Emperor ends right there, so we have no clue if there's even a chance a Dragonborn Emperor. All we can do is sit and wait for new lore relevant to this I guess.

And this is assuming the Dragonborn did the DB quest chain at all.

Perhaps. But I'm willing to bet 50 Septims that the outcome will be that someone (Dragonborn or some other equally talented assassin) will meet with success.

Having said that, there are plenty of enantiomorph possibilities in Skyrim and the main ones I can think of are better alternatives than Astrid/Dragonborn/Emperor, in my humble opinion.