I think it wouldnt be to hard to implement, since we already have Simple formation globalAI and Rightclick+Drag line.
Heres some concepts Ive made, these are just suggestions though. Most of which based off of Homeworld ones.
Delta

Directly copyed from homeworld (>_>...<_<...), all out useful formation, especially for planes.
Reverse Delta

Thought this one up a while ago, Instead of spreading fire on targets, converging fire on one target.
Horizontal

Its a line. Need I say more?
Arc

This would be a great one for gunships...
Circumfrence

This ones purpose would be for escorting a target in around it in a circle. Also, can be used for converging on one target from all angles.
Box

Its a box. Stfu, everyone loves boxes.
PS: I think the diameters should be customizable somehow.
Custom
Make your own custom formation, Isnt that what Simple Formation AI already does? Im not sure, never got it to work.
Gimme comments. And ill submit this to mantis.