I've finished a working product and we haven't touched it in months. Recently, there was talk of updating it. The issue is the game is getting boring. We are going to find two components that make it not so. Oh! Pardon me. I haven't even told you what the game is yet. We call it The Marble Maze, a game where you rotate a maze to your marble to the finish. On the way, you'll find spikes that will kill you, so avoid them! The first thing I thought was having a key and lock. If the finish had a lock, there'd be a key as well. There is, however, not anything worth while on the way. So, the biggest problem is, what do we have, other than spikes, that make getting that key interesting, or do any of you have a better solution.
How about Pac-Man style maze monsters? Maybe slower than those, but same idea... wander around the maze trying to catch you?
Could implement one-way gates. You can roll past the gate the first time, but then have to find an alternate route.
How about some ramps to make the ball jump around or even a thruster or pinball like bumper. You could also add things like score and speed leaderboards.
How can you run out of ideas on something like this? Monsters that chase. Time hack. Earthquake. Turbo mode. Trapdoors. Fire. Mines. Seagull poop. Invert gravity. Different surfaces like ice, rubber, toxic sludge. morph marble to different shapes to fit different size passageways. Special one use powers per level, like jump outside board. Smash through walls. etc etc etc The problem you should be having as a creative is having too many idea's to implement, and not enough time.
Sounds like the old wooden Labyrinth game. I'd suggest holes to fall through like in the original for starters. Could be done for phone or tablet where you actually tilt the device to roll the marble. Add various abilities or powerups, like a temporary shield to protect against traps or creatures, or something that operates like a point defense system. You can have all kinds of environment obstacles, water, fire, etc. You could have to move objects out of the way by rolling back, getting a good "rolling" start, and hitting them to move them out of the way.