I have been told of a couple formations, so I thought I would hit a couple of them. Here are two, one for Grey Knights called A Grey Knight Extermination Force, and one for Eldar, Wraithknight Dreamwalker squad.

Please remember that these are rumors until we get official information

Grey Knight Extermination Force
Consists of 3 Stormravens and a Dreadknight
The Dreadknight marks the target, and the Stormravens automatically hit it

Wraithknight Dreamwalker Squad
Consists of 3 Wraithknights or More?
They become energized by the more Wraithknights in the formation, becoming more accurate with shooting and in close combat, also increases to movement and charging.