The best ideas come when you least expect them. Don't try to force your brain to come up with ideas, just do things you'd otherwise do, and maybe you'll get good ideas!
I don't think there's a time minimum to how long the gameplay should be. I also have less time other people, have to go to the hospital twice and 2 days I am at my grandma's house, without my PC :(
I am thinking about making a game in which "the game" is a famous person and the plyer just found out where he lives, so he lies about the way to where he is, and locks the player up in the basement. But if you find out where he is... That's a surprise for if you play the game :)