I think you could handle those things pretty easily in an abstract system like the one I sketched out. You'd need one or two rules that I didn't explicitly mention in my first post for some of those, but nothing terribly complex.
A mage comes upon a group of zombies grouped around a fresh grave, feasting on the entrails of some hapless villager. Suddenly, one of them sees him, and lets out a low moan. In an instant, all the zombies are shambling towards the mage.
The GM surveys the situation and decides that since almost all of the combatants (the zombies) are grouped into a tight space, the scale of this battle is "melee," but the mage is initially only "close" to the zombies.
The mage goes first, focuses on one of the zombies, and runs away. The zombie is either too dim to oppose or fails its movement test, because it's a
zombie, so the mage is now medium away from that zombie. All the other zombies are "melee" from the zombie the mage focused on (by the battle scale rule), so by the inferred movement rule, those zombies are now all "medium" from the mage, as well.
If the zombies focus on the mage right now and each try to move towards him, the mage can "circle" to oppose
each movement, rolling a separate test for each one (unless the zombies are numberous/weak enough that we decide to treat them as a single "horde" unit or something).
Summoning his arcane power and lifting high his jeweled staff, the mage conjures a globe of fire that falls from the heavens upon the zombie horde.
The mage hurls a fireball at the zombie he's focused, which, as per the power's description, has an area-of-effect of "melee," so all objects within melee of his target (in this case, all the other zombies) are also affected.
Ice9 wrote:* If the zombies spread out, they can mitigate the effects of the fireball when it happens. But spreading out will slow their pursuit of the mage.
The zombies recoil and cry out as the fiery blast strikes them. Momentarily forgetting the mage, they scatter.
Each zombie focuses on the zombie that was just blasted with the fireball and moves away; the primary target does not oppose, so this is automatically successful, and each of the zombies moves from "melee" with that zomebie to "close." By the inferred movement rule, each of the zombies is now "close" from every other zombie and "medium" from the mage.
The GM decides that the scale of the battle has changed, and so the next round, he declares the scale is now "close" rather than "melee." Thus, the zombies will remain "close" to each other (rather than "melee") even when they focus back on the mage next round.
Ice9 wrote:* If the mage enters a tight passage, the zombies have a harder time following him and must stay bunched-up to do so.
As the ravenous zombies chase after him, the mage spots a tunnel in the nearby hillside. Seeing his salvation, he turns and sprints towards it.
The "tunnel entrance" is a terrain feature. It has the "bottleneck: melee" trait, which means that it segregates the battle area, and no one can cross from one section to the other unless they first come within "melee" of the terrain feature.
The zombies are now "close" to each other and "close" to the mage. The mage focuses on the tunnel entrance and approaches (and succeeds, since the tunnel entrance doesn't move very fast), putting him within "melee" of the entrance. The zombies, which were all focused on the mage, get their distance updated to their previous distance plus the amount moved, and are now "medium" from the mage.
The mage tosses a spell over his shoulder as he runs, but takes an accuracy penalty because he isn't targeting his focus.
The next turn, the mage can move into the tunnel, and the zombies will all need to get within melee of the entrance (and therefore of each other) in order to follow.
If the passage continues to be severely narrow, we represent that with a series of bottlenecks, so the zombies can theoretically spread out, but will always have to bunch up again to follow the mage if he continues retreating.
Ice9 wrote:* But if that passage has a dead end, the mage will eventually run out of room to retreat.
The mage runs out of bottleneck terrain features to retreat through. The only way out is through the bottleneck that the zombies are already at.
Ice9 wrote:* If the intial position of the zombies and mage was different (say, two groups, one on either side, but both are still "Close" range), the mage might not have been able to retreat as easily.
Two groups of zombies; zombies within each group are "melee" from each other, and "close" to the mage and zombies from the other group. The mage focuses on one zombie and retreats, as in the first example; the mage is now "medium" from all the zombies in one group, but still "close" to all the zombies in the other group.
Ice9 wrote:* "Tight passage" is a relative term - some passages are tighter than others.
So there's also a "bottleneck: close" terrain trait that works exactly the same, except you only need to be within "close" of the terrain feature to cross over, rather than "melee."
Ice9 wrote:* A passage can be made tighter with spells like Wall of Fire.
The "wall of fire" ability targets a "bottleneck: X" terrain feature, and any creatures passing that bottleneck must either close within X-1 range of the bottleneck first, or suffer fire damage as they pass.
Ice9 wrote:* A passage might not have a dead end per-se, but a tough foe could effectively create one by standing next to one entrance.
Probably works a lot like the wall of fire; the bottleneck narrows by one range step, but you can pass through at that range anyway if you're willing to give the opponent a free attack as you pass. (Other implementations are possible.)