To get better at writing game ideas i follow this small guide.
1: Use Mobygames random function to pick four existing games. The link to the next random game is two crossed arrows to the right of the sites menu links. I often pick games with a common element. Such as games release on the Amiga or games with some form of vehicle.
2: For each one write a three paragrah game pitch that use the selected game for inspiration.
Look - How do the player see it and what does it look like.
Do - What do the player do.
Pillars - Core elements of the game
3: Sort the games with the ones you would write more about at the top (A) and the least preferable ones at the end of the list (D).
4: Roll a D10 to pick one of the games.
A: 1-4
B: 5-7
C: 8-9
D: 10
5: Write a three page pitch with pictures about the game .