A few months ago some of the interns from Hex with help from @muqing_cao wrote ZigZag mode for Copter. It took a while but it is finally in master and will go out in Copter-3.7 (or perhaps a point release of Copter-3.6).
I’m pretty sure there was a blog post about this a few months ago but in any case, ZigZag mode is intended to make crop spraying with a drone a bit easier. It allows the pilot to use an auxiliary switch to define two points which should be on opposite edges of a field. After these points are saved, the pilot can fly the vehicle around freely in the equivalent of Loiter mode but whenever the pilot moves the auxiliary switch again, the vehicle will fly back to one of the edges of the field.
Hopefully the diagram below makes it a bit more clear. You can see Point A and Point B that the pilot has saved and also the edge of the field (blue dotted lines) shows where the vehicle will fly to when the auxiliary switch is moved either high or low.
Thanks to Cao Muqing and the Hex interns for this new flight mode!