famousgnu6
build remakes of existing games you think you can realise (start easy and enhance until it turns the wrong way - then reduce [and probably add other enhancements]).
e.g. start with a 'single player' pong clone (where the paddles move contradictory) and add some 'extras'
add some bricks in the middle - so you have a 'pong brick' clone
make the balls exploding (on some bricks)
make moving the bricks
make rotating the bricks
make the balls bound to layout (or make them warp)
add random appearing elements (spaceships, rockes, health, shields)
make the control behavior changing (direction/speed) while gameplay
in general think about games you like and try to combine parts of them.
If you can't imaging the resulting fun in gameplay just make a simple prototype to figure out.
start always with the programming prototype - good gfx is worth nothing if the gameplay is boring.
I know programmer love to see their idea with stunning gfx and artist love to see their gfx comming to life but starting with the gameplay saves a lot of time and reduce the frustration if your idea don't works.