u/Gastroid

As it stands, the following ethics + gestalts have an associated Fallen Empire:

• Fanatic Xenophile (Enigmatic Observers)
• ​Fanatic Xenophobe (Militant Isolationists)
• ​Fanatic Materialist (Keepers of Knowledge)
• ​Fanatic Spiritualist (Holy Guardians)
• ​Machines (Ancient Caretakers)
• ​Hive Mind (Shattered Fragments)

With the upcoming addition of Nomad empires, I think it would be a great opportunity to add a Nomad Fanatic Egalitarian Fallen Empire.

Their pops would be totally free. Free to travel, free from any borders. Sure, they lost control of their Arkships millenia ago and have no way to stop them, and their pops are all useless hedonists along for the ride, but that's besides the point.

Nomads will also have a special contract system, which they can make and receive, so a Nomad Fallen Empire strolling in to your systems with 1.5 million in fleet power and giving you contracts? You'd pay attention to that.