Just a random game idea I wanted to get out of my head...

I'm imagining a game like No More Heroes by Suda51, but the catch is you're walking like you're Darth Vader from Rogue One massacring the rebels. You have a FEAR METER, a small semi-circle in the corner of the screen that fills up based on actions, and changes color based on mood.
The gimmick is that as you stroll along and slice baddies to bits, you will add fear to THEIR fear meters. If they hit you, your fear meter will go up. You cannot go faster than a walk unless your fear meter reaches a certain threshold signifying growing unease, let's say at 50% fear. At that point, you'll be able to run at will, but that will cause enemies to realize that you're starting to get scared, lowering THEIR fear meter. If your fear meter reaches 100% fear, mission failed; you will automatically retreat from the area.
You can reduce your fear by getting back into the groove of hitting bad guys, but your health will remain lowered if you were hit, and you will also fail should you die; getting hit on low health will have a greater impact on fear than getting scratched at full health.
Killing enemies will raise their fear meter until it is full, at which point they will react differently; weak or injured enemies may run away, while some enemies will grow desperate and grab pieces of the environment to give themselves an edge. You can use the environment to your advantage as well; say, pick up a plant and chuck it at the enemy. Enemies can relax their FEAR if they receive reinforcements, land some good hits on you, or they get pissed at your unstoppable advance.
The Fear Meter will be yellow if your feel afraid, but red if angry, blue if saddened, and green when ecstatic. White indicates general neutral interest, but the meter can change between any of the colors at any time. You may receive a few hits and get angry at first, increasing damage until the meter goes back to white, or you may get a lot of damage and realize you're in big trouble as the meter turns yellow. Blue is mostly story-based and can reduce damage during key events, while getting 50% of the meter full at white can cause a sort of in-the-zone mood, turning it into green and allowing you to move a bit faster and hit a bit harder.

Tl;DR The fear meter is an idea for a hack-and-slash that changes based on performance and translates into in-game mood for both player and enemies. Said mood can cause enemies to change fighting style or run away, while your own fear meter can change according to your gameplay proficiency.