Spoiler:The idea is simple. In the WorldLoaded event ID_Point markers are placed at the center of each sector. I have kept this at the Sector property for now. Similar to other AI mods, these are monster waypoints. For the present all these are marked by a flat sprite and all monsters are flankers. The flank() function uses a ThinkerIterator to find all monsters within a certain distance of the player in a See state. Then another thinker loops through the ID_Points to find the first one farther away than the player and on either side of the player. If a point is found the monster attempts to move in that direction. For debugging purposes I apply a different RenderStyle when that happens.
I've just started looking at monster AI, and there are issues around map geometry, infighting, placement of monsters, etc. A player is seldom still, another significant variable. All the same, my early testing seems promising. VectorAngle() can select ID_Points in the direction of the player (a positive or negative angle), and I think AngleTo can find player flanks (I assume this depends on the player's current facing angle). I'm still playing with this, but the idea is to implement a general attack strategy that isn't head on.
Work in progress...