Screenshots:
Spoiler:The above screenshots show this in action. The top is a monster in "flanking" mode as a shadow running around the player. The bottom is the monster camera showing the monster behind and to the right of the player shooting away. (These can be seen with debug options turned on.)
Monsters become shadows, zip to one flank or the other, become visible, and start shooting. It's possible to change most of this from Settings, including the range of flanking, how many monsters flank (I don't recommend 1:1), shadows or not, sounds or not, size of the player's flank (larger is better), and sector marking algorithm. So far this seems to be aggressive AI, but (hint) I think it can be avoided by running away.
What it does is simple. Sectors are tagged with center spots according to one of three algorithms. A flanker will head for the nearest one to the player once the player is seen, preferentially one already flanking the player. Flankers will be non-floating, missile-attacking monsters.
Monsters will become confused in tight or angled quarters which are challenging to mark with a center. I assume this works better in open areas, but I'm still testing. The shadows can be turned off, but if they are on you can still turn as the player and see the flanker for a moment. Note that the debug camera operates from one flanker at a time, which helps.
This should be compatible with other AI mods, since it doesn't interfere with path finding algorithms. Flanking is only turned on at the beginning of a "See" state, and only for those monsters who spawn as flankers.
Updates here and there as I play and optimize. (It was slow with Head Boxes but otherwise these shouldn't make a huge difference.)
Update 12/20/21
Spoiler:Update 12/24/21
Spoiler:Now at v1.6
