Okay, say some spells over the years, we'll throw in illusory damage, too! We owe damage typing mostly to skeletons and trolls, I think, and it got extremely fucking out of hand for a while there. Even if you have 14 types of damage 'illusory' shouldn't be its own category, should it? Maybe just make it psychic damage...4e did a lot of that for fear and illusion effects that damaged...but next thing you know everyone is doing psychic damage and to me the words "psychic damage" should carry more weight at the table than Type #12. It should be an "oh shit run it's the Clefft Beyonde Starse pouring into this world and undoing the basic notions of unmadness" kind of thing, a sign post that you've found someone for whom the world is their dungeon and any ACTUAL dungeon they're in right now is water off a duck's back, as consequential as ultraviolet light to us.
The idea is sound, though: you think you're hurt, you may think you're down for the count even, like when you Command someone to "die" and they pass the fuck out. But once the duration is up they say "Wait I'm not decapitated at all" and rally. I think the only reason it seems so weird and fiddly a subsystem is that it IS a weird and fiddly subsystem. If it were a concept/tool in the monster box as common as say old-school petrification or new-school domain effects then nobody's bat an eye. "Of course illusions can do that, illusions are MADE to exploit that part of the rules!" Rather than the other way around, yes? And it's not like there's not a ton of examples of ways we can use it.
There's a little place on a lot of late-edition character sheets for Temporary Hit Points.
Do what they say on the package: you have these HP for a while and then you don't. This can act as a kind of half-heal or a little shield, a buffer before you take REAL damage, like rings in the 1 1/2 Sonic games that's not entirely bullshit. The fluff justification for this is always a race, class, spell, feat, or item effect which confers this bonus, and there are some common expressions: berserker rages, drinking, empty-self serenity, mind-over-matter, some holy invigoration, drug, magic, possession by an elder spirit, inspiring words of encouragement or wisdom, being a plain old tough bastard, good old hodad magical crap, the ancient blood of your ancestors, just being from SPACE or a robot, the list goes on.
Now this isn't traditionally thought of as an OSR friendly mechanic but it jives with my impression of hit points as kung fu, using some technique or other (it hardly matters) to draw on an inner reserve to keep you up until you've saved someone/gotten revenge/found your way home/whatever. The current edition of DND doesn't use this feature as much as the last few but it's still a core feature.
So, lately, I've been thinking about Temporary Damage. I think it's a damage category more useful than which Mega Man bad guy weapon the damage is typed to, usually. A spell effect, environmental conditions, disease, toxins, whatever, affects you physically and de-centers your chi, making you weaker and weaker. Eventually, if Temporary Damage would take you below your current HP threshold, you are rendered incapacitated, unconscious, and helpless, so that someone can deliver a coup de grace. OTHERWISE, left to your own devices, your body-mind will repair itself and you'll pop back up hale and hearty with a few hours of recuperation.
There's lots of situations where this would make sense. Lack of oxygen, for example, or a lot of things recent editions have used psychic damage for.
What makes something like illusory damage seem weird and annoying to track is because it just sits there on top of all the other systems of damage and saves. I say make Temporary Damage core to the components of the machine, and illusory damage just one facet of Temporary Damage. I mean BX already has a Temporary Damage mechanic discussed in the section on taming dragons: you beat their ass with the flat of a blade but don't stab or cast spells at them, and track their HP on a separate "whip you into shape" track. It makes sense that this works for other things like owlbears or T-Rex, too, so I use it for that purpose and I'm sure other people do, too. Why not just codify that a liiittle bit more so we've got somewhere to make things like illusions and POISONS (oh my god the horror stories of the pretzels some tables twisted themselves into over the use of poisons?!) easier to use?
I mean, I'm gonna.