Main idea: A treasure-hunt platformer that takes place in space.
Secondary ideas:
-The player starts in an open-world space in the ufo gamemode, and can freely fly towards any visitable planet to land on it. There are 3 main planets, as well as 3 smaller rocky planets which have one user coin each. There's also debris and asteroids which will kill you upon impact.
This idea probably takes the lowest priority as it's essentially a glorified level select screen. It's best to work on the levels themselves first (with a much simpler temporary select screen)
-The main goal is to find 10 minerals, 3 per planet, + one floating in space. The player has a radar that beeps when getting close to a mineral, the more frequently it beeps, the closer the player is to the mineral it's detecting. The radar always detects the closest mineral and does not detect the user coins.
The radar mechanic takes the highest priority as it is part of the main mechanic of the level. The beeping range could be reworked later to fit properly with the layout but it's best to have it working first.
-Each level is an open area that starts in the robot gamemode, with no paths to take, and hazards all around. Certain mini areas have their own gamemode. While the layout of these levels stays exactly the same everytime, the places the minerals spawn in are randomized. The three main planets take the themes of: a mountainous desert with getting-over-it style gameplay, a half-submerged land with an underwater maze in the ship gamemode, and a forest with an El Dorado-style temple and many hazards.
The random spawn mechanic would need to be developed either before or in tandem with the layout.
-Once all the minerals are obtained, the player can land on Earth to end the level.
This is simple and can be done last.
Tradeoffs:
Optimization and quality take priority here. As long as the end product is good and can run on most devices, how long it took to make shouldn't matter.