There should be an option to make sampleDistance be fixed angles around the player. So if I input 45, 90, 180, or 360 it limits the spawn location of that mob template to specific cardinal directions.
45: limits spawn location to 8 cardinal directions
90: limits spawn location to 4 cardinal directions
180: limits spawn location to 2 cardinal directions
360: limits spawn location to 1 direction

And an additional variable that offsets the angle AFTER the sampleDistance angle is set would allow invasion developers greater control of where mobs spawn. For example:
Normally, setting sampleDistance to 90 will spawn mobs from the north, east, south, and west directions. Adding a sampleDistanceOffset of 45 should then spawn mobs from the north-east, south-east, south-west, and north-west directions.
Similarly, while a sampleDistance of 0 or 360 will always spawn mobs at angle 0, an offset of 90 should always spawn mobs at angle 90.
This would allow invasion developers to set the spawn location of mob templates essentially anywhere they need, and allow for waves to focus their numbers from certain directions rather than being spread out.