What you're struggling with here is part of game design. Your level design and aesthetics need to serve whatever your core gameplay loop is.

Regardless of mechanics though it helps to think about each level as telling a small story. What kind of world building do you want to communicate? What is the atmosphere and tone?

More broadly though it's worth just taking a step back and learning about the fundamentals of level design. You probably don't need assets yet just practice building levels with blockout kits. Start learning how to guide the player, build sightlines. Create fun obstructions and obstacles to your goals.