As a case in point, many games have a certain "reactivity" when it comes to the game world: townspeople react when you clear out the kobold caves, or they'll offer a tidbit of customization if they've talked to you before - "Oh, it's you again. What do you want now, <charname>?" -- but that's usually as far as the reactivity goes.
The Orc Army doesn't attack Inmesvale or set up camp in the Ruined Temple of Amaunator, if you've killed Firkraag or defeated the Shade Lord, respectively.
The instances of such reactivity need not be large, nor do they need be particularly well tied to the plot, nor would they even need to have far-reaching consequences. They need not be as complicated to program as a full-fledged romance, either. If you're lucky, you'll get a case where the army invading the Seer's camp in the underdark
won't have mind flayers with it, if you've cleared out a certain area; very rarely can I think of a case where the reactivity of the game has been more proactive in nature.
That, I think, might be something worth exploring, if I were designing a game. I know the game is going to manipulate my actions: the more heavy plot-driven the game is, the more I'm going to be hemmed in, somehow. I accept that. If instead of giving me the feeling that it was merely the game designer doing the manipulating, though, I could feel like it was the antagonist that was doing the manipulating, I'd get a nifty little world-building illusion and an actually clever-seeming villain to boot, instead of the now-cliché "I'm too powerful to care about you, but when I do eventually realize that you're a threat to me, it'll be too late" passive Foozle. Mephistopheles in HotU was a manipulative villain, certainly, but it was utterly passive in relation to the PC. He never actually did anything to my character, or did anything in response to my character: his actions were set in stone no matter what I did. Every other game villain I can think of is exactly the same way.
One would have to choose one's events with care, I'd imagine, for subsets of reactions to all make sense within the current framework of the overall story, but I think it's something that could be done, particularly if such sets were kept small and their effects relatively localized...
... if one was interested in presenting a more reactive experience for the gamer. I don't see much evidence of that in the industry at present, though. If anything, story-driven games are tending more toward a movie-like experience.
Personally, I can still enjoy the regular Passive Foozle, and while the feeling of being manipulated by game designers, instead of something In Context, can be annoying, that feeling is mitigated by the inescapable knowledge of what I'm doing: Playing a game. I know it's not a real role-playing experience; it's a pre-programmed piece of software, and no matter how well they disguise it, there are only going to be so many things that I can do, and only so much influence I'm allowed to have. That's all right with me.
For now.