Angel to Devil would have been great.

[Vorgor]

It's kind of sad it won't happen but it could have been pretty cool. It could have been unlocked by being more lawful than good and add the option to corrupt the other angels and Galfrey.

[Ishan451]

Aeon is the Lawful Neutral option. When you want to be more Lawful than good, then Aeon is the Path you need to take. Aeon's are creatures of embodied Law.

Also Pathfinder Angels / Devils have nothing to do with Christianity. Angels (Being of ultimate good) and Devils (beings of Lawful Evil) are quite different things. The Evil Opposite of an Angel, in Pathfinder, is a Fiend, not a Devil. The Good Opposite of a Devil is an Archon. Archon's are "lesser" than Angels. Just like Devils are lesser than Fiends. Though you can have an Archon Angel and a Devil Fiend. As they are both also kinda creature templates and Metaplanes are weird.

I also don't think Angels can fall, just like Fiends can't "ascend". If they do, they usually change what they are. See Nocticula and Lamashtu. Admittedly Asmodeus remained in Hell after achieving Godhood, so that is probably why he still is a Devil... or his Godhood allows him to remain a Devil. But the other two left their Demonic Nature behind. And the former arguably is trying to climb over to the "good" side.