Theme: Atmospheres
My idea for this level is having two parts, one being a sky area, with rain, thunder, snow and storms. And the other being a space area where you get lifted by a rocket and fly through some planets, stars and meteors while having enhanced gravity physics.
MoSCoW method priorities:
MUST: Space, sky.
SHOULD: Storm, Snow, Hail, planets, stars, shooting stars, rockets.
COULD: Thunder, Aurora, Hot Air Balloon, Satellites, Rain, clouds.
WOULD: Planes, Asteroids, Meteorology Instruments
I would choose Quality and Optimization as the tradeoff because I like quality and good levels (and i know most people do too) even if they take time, because it's worth it!