Enemy with spotlight looking for you. Level is darker, close pillars. When light sweeps over the enemy freaks out and starts shooting wildly.

Fade sound based on proximity

Attract mode showing enemy types.

Enemy codenames: wall-e, bomb-r, form-l, snip-r, foun-t, bria-n, spot-r, hose-r

Enemies enter with distinct sound, or no sound for some.

Prompts: Get to the portal!

How do shots fizzle out?

Multiple suns, multiple shadows

Low sun, sunset world

Skyboxes with suns

Effects

Things that wobble and shake feel organic and/or scary
Particles like: smoke. Fireflies. Wind in a direction. Snow. Rain. Or one general effect for depth and texture. SPARKS.js
Ease down the clock multiplier for a very cool slow-mo effect. This will break clock-elapsed timers.
Camera move: use ease in/out for move/rotate cinematic sweeps - tween js does this.
Very slow rotate/pan is cool for a 'static' but live menu screen

Sound