Just now I remembered a particular game that I planned when I was young that I'd really like to do now, or for the possible-future Childhood Game Compo. The game was all based around a single concept, that the player would never be allowed to rest. Through the whole game, they would always be being chased by something, or on some kind of unstoppable vehicle, or fighting a boss, or being shot at/attacked, or any combination of these. I guess this concept is already in effect in certain rail-shooter type games, but this would focus much more on running away than attacking. What brought back memories of it was this section in Abe's Exoddus where you have to run through a series of screens, being chased by four, five different types of enemy. Every time you escape one, you end up dropped in front of something else.
In my original plan for the game, it was called 'Nebula Starfalling', because the story was that all the stars were exploding for some reason, and you had to stop it. I planned out this elaborate sequence in which the player had to fly a glider through a canyon, using whales' blowholes to give him lift, while pursued by a giant flying manta ray, which eventually chased him into a stone temple that was gradually demolished and filled with water, until the huge stone spire fell and crushed the manta ray boss. Then the player had to ride the manta ray's corpse down into an underwater cavern.
That sounds ridiculously fun.