I have mixed feeling on the Aasimar. The mechanics are amazing, but the fluff feels like a retcon from the SCAG.
The Majority of Aasimar, the place where they were most common, is Mulhorand and the surrounding lands. Some would have connections to Mount Celestia in other lands, along with other higher planes, but in Mulhorand and surrounding areas the Aasimar are descended from Mulan Gods and their celestial servants.
Otherwise its great. Still for a nominially FR book it seems to have next to nothing connecting it to the Forgotten Realms so far from the previews, and the promised Elminster and Volo sparing seems to in short supply so far, but the previews on this front could be misleading.
But as I said I've only the previews to go on, so maybe they are misleading.