All games (in their simplest form) are just moving one object to another object. The way we make it into a game is
- Theming: Instead of collect the orbs. Collect the keys to escape this labyrinth!
- Obstacles: A player must have some sort of obstacle to their goal, that they must overcome. For instance the monsters movement is an obstacle. A good way of making that more interesting is making it less predictable having the player think on the fly.
- Gameplay loop: A gameplay loop is all the actions a player does in order to play the game. In Minecraft it is gather resources in new places -> build tools to explore new places -> gather resources. Although that may seem simplisic it is the obstacles and challenges which modify the gameplay loop and have to make the player think on their toes. "I can't go in the cave because there are zombies, so I have to destroy them!"
I have no idea if this helps, but listening to other game devs will help your ideas too!