No complicated input like WASD, ctrl, alt, space, XYAB etc... What interesting things can we do using just the mouse and left click? (eg. Peggles, Bejewelled, Fruit Ninja).
These are continuous games that gets progressively harder. No levels. It's called a 5-minute game because the average player usually dies at the 5-minute mark, due to the escalating difficulty.
Games that focuses on thinking, and solving.
Take classic game concepts, and change their mechanic.
Let's target ages 6-12.
These are intended for mobile devices as they may use features such as gyroscope, accelerometer etc... You can play on the PC but with simulated controls for those.
These are targeted to preschoolers. Any writers out there?
Pick a random verb. Create a game that uses that as a game mechanic.
Pick a random noun. Think of the verbs you could do to the noun. Use that as a game mechanic.
These games focuses on the audio aspect over the graphical aspect. Bonus if the visually impaired could also play without disadvantages.
Games where multiple people can play, but on the same computer (think simple board games etc...).
Games that can have positive social impact.
Anything. This is for those interesting ideas out there, but there isn't a category. This category is spread across the months as fillers.