Now, to make things clear, and direct this discussion towards the technical part rather than personal problems :The idea simply put, when any character (PC or NPC, with some exceptions) gets below 25% health they fall unconscious. This could be the same point low health scripts (where npc say things concerning injured comrades) kicks in, making for a much more dramatic effect.
Undead, trolls and a number of others would be immune to it. Raging units could be immunity to it (but lose the temp bonus to health they get)
The more ways to deal with a situation other then killing, the more roleplaying involved. If I can knock a npc out rather then killing it (or ignoring it as it attacks me which is really silly of course) then that gives me another tool to deal with a situation and mod story tellers another option at their disposal.
There are numerous problems with your idea TruePurple, as I will list them below. That is why I said it is forced. It is not an offense at all, I had my own share of poor and forced ideas back then (and even nowadays )...
1. First of all, many creatures are immune to Uncounsciousness already. Some of them by their nature, racial traits, or merely to make them tougher in specific encounters. In ToB this is even more so. Now, to implement your idea, one would need to alter these creatures, and sometimes mess with their basic concepts as well - not something that would be a good idea if you ask me.
2. The second problem is that there is a great difference in that 25% when it comes to a Hobgoblin and a Gragon for example. While I accept that the first falls uncounscious when reaching 8 HPs, I can hardly accept that the Dragon does the same at 50 HPs. %-s are never really fitting to be used for this. Perhaps if the limit would be toned down to 10% it would become more believable.
3. The third problem comes with all the current low-HP dialogues of opponents. Many foes have specific dialogues when they reach a specific HP amount (or using the MinHP1 item), but their scripts wouldn't be triggered if they'd lie uncounscious - you'd have to alter these as well.
4. Your idea to see opponents fight more tactical (e.g. healing, reviving the injured/uncounscious comrades) sounds promising, the only problem is that these need to be scripted as well. And that would mean much work, again. Opponents already do this in a few ToB encounters (Yaga-Shura battle for example) where they give potions to the wounded comrades for example. But that is only one fight from the 10.000 in the game - do you have the time and patience to script the others?
And I had other probelms as well, but they escape me right now. Either way, this is not something that could be perfectly incorporated into THIS game, no matter how much work you put into it. Where I sad the engine is limited in this regard, I meant it.