Except Alessia didn't invent the name or concept of Akatosh. That name was taken from a pre existing Aldmeri concept according to Shezarr and the Divines. In fact it wasn't Alessia at all who tampered with time and the Prolix Laws. That happened a long time after her death.
Alessia's conception of Akatosh is diametrically opposed to the Altmeri worldview.
Her Akatosh stands for the preservation of Cyrod and the Mundus itself. This is a big change. Shezarr and the Divines seems to suggest that Akatosh was a member of the Ayleid pantheon, which makes them Altmer oddballs. It makes sense, actually, with their Tower magics and Daedric pacts, that they did not share the Aldmeri distaste for the mortal world. Now, once you realize that the preexisting Akatosh was just the cultural Time God variant of a sub-civilization of Altmer, arising chronologically after the settlement of Cyrod by Topal the Pilot's successors, you reach the same conclusion. The being we call Akatosh is no more original or true than Alkosh or any of the others. Alduin is definitively NOT the son of an Ayleid Time God.
The biggest deal is that, operating as the Female Principle, Alessia reconciled--for a moment--the enantiomorphic brothers of Auriel and Shor. Both deities forged the Covenant, in the person of Pelinal Whitestrake, an avatar of both.