Some enemies ought to be stealthy, rather than just rushing at you. Some might use illusion, others may use the shadows. Stealth becomes important with characters like vampires. I definitely think the AI should be updated to allow NPCs to use stealth intelligently.
As a thought on stealth, and enemies using it, the chameleon effect might be useful there. As in, the model becoming faded, gradually going from opaque to invisible. If the player had some kind of skill/stat related to "awareness" (probably the Tracking skills, in regards to my own set), the visibility would change according to awareness, NPC stealth skills, and proximity. A skilled assassin could press still against the wall and be mostly transparent. In a dark cave, nearly invisible. Improved AI would let stealth-based attackers automatically seek shadows or cover, and keep still when you're looking at them, preferring to move in from behind. As a graphical aside, maybe leaving stealth mode would cause them to rapidly "solidify" instead of instantly, making it less jarring than them simply popping into existence for those with low awareness. Some good AI would allow for things like NPC's making use of camouflage or being aware of footstep volume. Behaviors like this would be nice for both enemies and allied sneaks.
In general I'd like the threat of assassination or a sneak attack to be an
actual threat. As it is, enemies can't use stealth, combat involves many hits, and when you're sleeping you rise into a fully awake ready position instantly, fully armed. Yeah, don't mess with the Dark Brotherhood, they'll...charge right at you like any other enemy. With real danger from a sneak attack, and stealthy enemies hiding in dark corners, being targeted for assassination would be an actual threat to your life. Even better: they quietly break into your house and poison your food. For "unusual" enemies, I'd like even more creativity. As they are now, vampires are hardly any different from ordinary human fighters with glowing eyes. Screw that, I want them to be in stealth mode while
clinging to the ceiling. I assure you, hearing that a cave might have vampires will be an actual reason to stay away after the first time one of them shrieks and hurtles into you from above, knocking you to the ground as its hidden allies swarm and tear you to pieces.