Given creatures are at full offensive strength until they die, focus fire guy is too often the best strategy. I figured there should be something to help spread things out and make things more cinimatic.
So.. inverse ninja law
When ever a creature attacks another creature, that creature get's a +1 bonus against all attacks and saves from any other creature. This bonus last until the start of their turn and stacks with itself.
So getting hit with 4 creatures, means you get +4 against the 5th.
That's... kinda OP considering PF2e's tight math. It's a cool idea, though I'd probably cap it out at +2.
Okay, not sure exactly what your goal is besides having a fun name for a houserule. I don't think you'll like the results, though. How broken this is depends on whether it affects PCs or NPCs.
If it's protecting the party: Mooks (monsters/NPCs more than a level or two below the party's level) will be way less effective. They already miss a lot, and once you get past the early levels, they already feel pretty much like inverse ninjas. Without focus fire on PCs, their impact will be lessened. Bosses supported by mooks will suddenly feel less dangerous if they attack after their mooks but not if they attack beforehand, which would just be weird. They can still just attack someone else, though, so they'll probably still be strong. Bosses without mooks will be unaffected because the bonus won't apply (though see the next paragraph).
If it's protecting the NPCs: Mooks will mostly just be targeted separately and will die a little slower, ironically making them live longer and likely make them more dangerous. Bosses, especially anything 3-4 levels above the party, will probably TPK the party more often than not. These guys need to be ganged up on.
Basically, the math is already such that anyone several levels below their target is already at a huge mathematical disadvantage in a way that compensates for the focus fire effect. Adding an additional modifier there will exaggerate the effect.
I think this is just going to make fights of PCs against mobs of low level enemies even easier for the PCs and fights of the PCs against a single boss enemy even harder.
I think I get what the OP is trying to do.
Imagine a fight between the classic party (Fighter, Rogue, Cleric, Wizard) against two giants. The best tactic is to focus fire on one of the giants, hoping to bring it down as soon as possible, while the other giant is more or less ignored. To me (and probably to the OP), a different pattern where the Fighter engages a giant while at least another character keeps the other enemy busy would look more interesting, heroic, cinematic.
Groups of intelligent enemies (or even animals, if they are pack hunters) should probably do the same: concentrate on one PCs at a time. It could feel a bit unfair to be ganged up and taken out of combat, even if that's what would happen even in real life sometimes.
If the six orcs with axes and crossbows you are facing all go straight on the Wizard, they are probably more effective, so a GM who for any reason doesn't want to run them that way would feel that they are pulling their punches.
That said, the proposed solution doesn't make much sense, and as many have said it would make 'bosses' even more lethal.
I don't know what would work, maybe something like this: if you were hit by an attack or failed a saving throw since the end of your previous turn, you get a penalty. It would encourage groups to spread their fire at least a bit, and make aoe attacks stronger.
I think I get what the OP is trying to do.
Imagine a fight between the classic party (Fighter, Rogue, Cleric, Wizard) against two giants. The best tactic is to focus fire on one of the giants, hoping to bring it down as soon as possible, while the other giant is more or less ignored. To me (and probably to the OP), a different pattern where the Fighter engages a giant while at least another character keeps the other enemy busy would look more interesting, heroic, cinematic.It depends on the enemy, right?
Ignoring considerations of engaging a "free mover" to protect your back line:
Western armies in 1980 were taught to shoot to wound - spread the damage out. The wounded one will be tended to by others, and you "increase your action efficiency". In Pathfinder, hit one hard and the Cleric switches from damage to healing. 2 less swords smashing you. In 2020 the enemy is different and many weastern soldiers are now taught to shoot to kill - concentrate the damage in Pathfinder terms - because the enemy don't evac their wounded so much and it also sucks being shot in the back by someone you thought was out of it.
edit: The analogy does not exactly translate because of the different hit point systems in use.
Of course it depends on the enemy. When every combat follows the same pattern, it quickly becomes boring.
But with the current mehcanics, even if say, the Cleric, engages the second giant, it should probably ignore them and go for the Wizard anyway, or flank the Fighter.As you said, since wounded PCs or enemies still fight at full efficiency, chopping away a few HPs doesn't stop them doing whatever they want. You have to either kill them, or bring them low enough that they consider retreating - or some of their team switches from offense to healing.